Internal Fragmentation and External Fragmentation.

In the memory allocation model, if the memory is partitioned into the fixed size blocks[M bytes a block], then when a process(N bytes) is allocated into the memory, it will fit the first N/M blocks, and occupy the part of the last block, and this is called "internal fragmentation", since the fragment is internal to a block.

The other model is called "external fragmentation", the memory is NOT partitioned into the fixed size. It will maintain a free list of "available" memory blocks and select the best fit for the waiting process. The fragment is external to any occupied blocks so it is called external fragmentation.