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USB drive is a small storage device after floppy disk and CD. Use of USB drive is much easier than floppy or CD. Data transfer speed is also fast with USB drives. We have to just plug it in to USB port of computer and we can transfer information/ Data instantly. We can use it for transferring data for one machine to another. This is very small in size so that much easier to carry with. In size this is as small as pen so as easier to carry as a pen can be.
This is almost completely sealed so that its data becomes safe from external magnetic field and dust. In USB drives there is a small PCB (Printed Circuit Board), inside case. So the small circuitry becomes safer than other storage media. It consists a flash memory for data storing purpose.
USB drives are available in a wide range in the market from where you can purchase it according to your requirement. The USB flash drive consists of a NAND type flash memory device for data storage which is integrated with USB interface. This is removable and re- writable means we can write data on it. In size this is much smaller than a floppy disk. Range of USB storage capacity is from 64 MB to 64 GB with improvements in size and price / Gigabyte.
It has many more advantages as durability, more storage capacity, handy, removable, re-writable, fast operating, easy in use, less weight etc. than other storage devices. So we can say that USB is a revolution in the storage media.
The SD(Secure Digital) card was introduced by Scan disk corporation, Matsushita (Panasonic) in 2001, it is based on MMC(multimedia Card) differences between MMC & Sd card are SD card is slight thicker in size,& has a write protection switch. MMC cards can be used in all SD memory card slots but SD cards can ot be used in MMC card slot.
Now a days SD card is most commonly used memory card. In size SD memory
card same as Postage stamp (32 × 24 × 2.1 mm). we can find the SD memory cards in so many electronics devices as Digital camera, PDAS, Mobile phones etc. The general name of many expansion modules is SDIO (secure digital In & Out). After inserting in a slot SDIO modules can be used for many other functions such as Bluetooth adapters, GPS receivers, digital cameras, TV tuners, etc The SD card s are of the following types as Mini SD card, Micro SDcard .
The Mini SD card (21.5 × 20 × 1.4 mm) was introduced in 2003. It was the Second & a smaller version after the Secure Digital card, this is approximately 60% small in size than the SD card & having same benifits like SD card except write protection switch.it was designed for portable electronics devices as mobile phones, Digital cameras.
The smallest version of this family is Micro SD card (11 × 15 × 1 mm). It is also Known as transflash. it was introduced as smallest memory card of the wold in 2004. the size of this card is approximately of the fingernail , It is basically designed to use in mobile phones. Today Micro SD cards commonly used in mobile phones.
Due to the development of various DRAM technology, it is now becoming difficult to determine what type of DRAM should you use in your system. Moreover, we always keep thinking "what is the next?" that would be more useful in terms of performance and price.
It would be interesting to know that system chip set is the most important factor that controls memory used in the motherboard. There 
can be difference in chip set too, like EDO and BEDO. The EDO is popular and accepted by all, where as BEDO is never gained popularity. It is the documentation of motherboard that indicates the type of memory supported by the board.
Most of the people wants to use a memory that helps in running the system faster and is cheaper too. Newer technologies are coming up with significantly better performance. It is often seen that people unnecessarily buy new memory to enhance the speed of the system though they do not have much requirement. Before buying a computer memory we should determining our usage and also analyze the performance of new technology DRAM. It is useless to buy newer technologies DRAM that are cost significantly more than older ones and offer only marginal performance improvements.
But the advantage of using a newer technology type will often give you more flexibility for future upgrades, since you may be able to transfer the memory more easily to a newer motherboard. However, this is not always the case, since often, new motherboards require a new type anyway. Also, sometimes new technologies don't catch on (for example, BEDO). The "latest and greatest" is of course often a lot more money.
With the arrival of DDR3 in the market, things have come down for DDR2. Everything is fine with DDR3 except the fact that it is very expensive and not many people wants to buy it paying higher price. This is still making DDR2 popular and favorite amongst the user. It is believed that DDR3 has a very little advantage over DDR2.
Moreover, branded names like Kingston, Hynix, Aum, Crucial are coming up with new and powerful DDR2 kit which can overshadow DDR3. The price of these newly launched DDR2 kit will be comparatively lower than DDR3, this would definitely attract a huge number of user to use the product.
Following is the Features of both DDR2 & DDR3, read it and give reviews which one is better. It is ultimately the user who would decide which one is better in terms of everything ( performance and price).
Features of DDR2
1. DDR2 SDRAM - High-density memory that achieves high-speed operation and low power consumption at the same time
2. DDR2 SDRAM - High-density memory that achieves high-speed operation and low power consumption at the same time
3. DDR2 SDRAM achieves a data transfer rate of up to 667Mbps, double the data transfer rate of DDR333, and its 1.8V operation provides lower power consumption than DDR SDRAM. Additionally, DDR2 SDRAM is available in an FBGA (Fine-pitch Ball Grid Array) package that saves space in the system and enables fine signal integrity.
4. DDR2 SDRAM realizes high speed and high density for a high-performance system.
Features Of DDR3
1. The DDR3 components are twice as fast as today's highest speed DDR2 memory products.
2. The first computer systems equipped with the advanced DDR3 memory technology are expected to arrive in 2007.
3. The main advantages of DDR3 are the higher bandwidth and the increase in performance at low power.
4. The DDR3 SDRAM devices will offer data transfer rates up to 1600 Mbps (megabits per second).
5. The supply voltage for the memory technology is being reduced from 1.8 volts for DDR2 to just 1.5 volts for DDR3 targeting a work day equivalent of battery time. The voltage reduction limits the amount of power that is consumed and heat that is generated in connection with the increase in bandwidths.
For average users of pen drive, the device is a simple storage device and they rarely bother about the features so long as it solves their basic purpose of storing and transferring information. But the tech-friendly and discerning users of pen drives are more careful about the features.
Today, pen drives have come up as the most advance and effective medium of data storage and have become a popular tool for business and professional people. USB drives have established themselves as the most effectual storage device, which allow users to store a vast amount of data without much efforts. This is because of the increasing popularity of these useful pen drives that CDs and floppy diskettes have been replaced and have become a second option for storage. Invented in 1998, these drives are re-write able and movable. The most fascinating feature of this majestic device is that it holds memory without any power supply. Moreover, it is very easy to carry it anywhere.
Sporting all the convenient features of a removable storage device - featherweight, small size, larger storage space, and durability - pen drive is a must-have device for the technology.
Though the basic selection of USB drive is based on prime feature of storage space, technical enthusiasts are developing special interest for advanced features of pen drives.
Apart from the generic features of huge storage capacity, pen drives also feature built-in micro camera, voice recorder, biometric security, MP3 playback. Let us discuss about some of the basic as well as advance features of pen drives.
DDR2 is a type of an SDRAM, this computer memory stores its memory in memory cells. These cells are then activated at according times to correctly operate with an external data bus. Although the DDR2 shares many similarities with the original DDR, as in the case with data transfer happening with the rise and fall at the edge of the clock, it is different in that the DDR2's bus is clocked at twice the speed. This basically allows the DDR2 to be twice as more efficient than the original DDR.
The key behind the high bus frequency with DDR2s is the coalition of electrical interface improvements, prefetch buffers, and various off-chip drivers. This is because DDR's prefetch buffer is twice of the DDR, which is 2bits deep. Also, its read latencies are between four and six cycles; again this is twice that of the DDR. Thus, twice as much bus speed is necessary to achieve an equal latency.
Also, the DDR2 is packaged relatively more expensively and more complicatedly than the DDR SDRAM or the SDR SDRAM, in exchange for a more blazing speed.
Backward compatibility with DDR2 DIMMs are quite limited. In fact, they are not designed to be compatible with DDRM DIMMs at all. This is because firstly, the notch positions in the DDR2 and the DDR are different along with the pin density slightly higher on the DDR DIMMs. Although not much of a "backward compatibility," but faster DDR2 DIMMs can be used with slower DDR2 DIMMs. However, in such a situation, the faster one would run at the speed of the slower DDR2.
Source :: Tech Faq
