Each Track is made up of one or more blocks. The AmigaDOS format uses 11 blocks, and PC/Atari ST (”generic hardware MFM (Modified Frequency Modulation)“) formats use whatever number of blocks and sizes are programmed into the controller.
Blocks are used to reduce the amount of data stored and encoded in one go, thus modifying just a small amount of data does not require a whole track to be modified, and errors on a block does not render the rest of the data unreadable.
There are disk formats used on some games that use only one block per track. This is a bit of a silly thing to do if you expect it to be readable later.
Blocks are normally separated by gaps.