Roughly, a USB flash drive is a device to store data in a flash memory with a USB interface integrated on it. This way the memory will be rewritable and also removable as a disk, but it is much smaller, will weigh much less and it is more comfortable to carry.
The storage capacity if a USB flash drive can vary a lot. They range from some mb to up to 256gb and every day passing they are larger and cheaper. The best USB flash drives allow to up to 1 million of cycles ( the times you can erase and write them) and may store the data with no errors for years.
The main function of these USB flash drives is the same intended for floppy disks in the past, but the advantage is that they are much smaller, the capacity is much higher and they are safer and more resistant. A typical USB flash drive won’t break if it falls to ground and will not get scratches as a common CD or DVD. At it is very little it can be stored in a pocket.
USB flash drives have a limited write / delete life so it is not recommendable to waste their life with unnecessary procedures like defragmentation. USB flash drives may be defragmented but it is barely useless because there are no mechanical parts in the drive and thus no mechanical head will move from a fragment to another. It may offer a little advantage because will need to access to less sectors but doing so will shorten the life of the flash drive in a great amount.
A USB flash drive can be partitioned as a Hard Drive but again it has little utility because installing a system in a USB flash drive will make too much writes and deletes and the life of the drive will be shortened.
The main disadvantages of an USB flash drive are for example the limited number of cycles before the device will fail. This may be considered if you plan to use a USB flash drive to run software or to use it as an OS. Another great disadvantage is that most USB flash drives do not include a write protection switch and thus, they are sensitive to be infected with a virus very easily. Their size is very convenient but on the other hand it is very easy to misplace them or leaving them behind.
About the price, they are very expensive if we compare to HD’s in the per unit storage cost, but when it comes to small capacities like for example a 8gb drive, they are much cheaper than a 8gb HD. They are very convenient for small capacities and to be used to carry personal data at all times like pictures, music, videos, resumes, documents and even medical information in the case of an accident because of their high resistance to water, impact and other inconveniences although they are not designed for this purpose.