To help you understand, a quick lesson on discs.
Windows hard discs have the data written to them in concentric circles from the centre to the outside; each of these circles is split into segments, each one called a block.
When you want to read some data from the disc the arm carrying the read/write head has to be moved to be over the appropriate circle; the system then has to wait until the correct block comes under the head when it can read/write it. Compared with the clock (calculating) speed of the computer this is a VERY VERY slow process. If all of the file you are trying to retrieve is in one block that's fine; if it takes up more than one block this process has to be repeated each time a fresh block is needed. It is quicker if all the blocks are next to each other as the read can then be done with just the one seek. Because of the way data on disc is deleted and re-used, after a while the disc gets very fragmented and needs a defrag to collect all the empty blocks together. All versions of Windows need regular defragging - it's just that the later versions mentioned by stoofur will do it automatically for you (if you let it).
An SSD is used differently. There are no moving parts and all the blocks can be accessed directly, so there is no waiting for a read-arm to move in/out or wait for the disc to rotate to the correct place. This makes reading much quicker and defragging completely unnecessary. There was a belief in the early days of SSDs that there was a limit to the number of times they could be written to. Advances in the hardware of the SSDs and advances in the way they are written to has made this limit to be not significant.