What is fragmentation? Different types of fragmentation?

Fragmentation
Fragmentation occurs in a dynamic memory allocation system when many of the free blocks are too small to satisfy any request.
Types of Fragmentation
(*)External Fragmentation
(*)Internal Fragmentation
External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much externalfragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous.
Internal fragmentation is the space wasted inside of allocated memory blocks because of restriction on the allowed sizes of allocated blocks. Allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used.