Disk fragmentation is the problem of files being stored in non-consecutive parts of a disk. Fragmentation takes place over time as file are added, deleted and amended. The file system tries to keep files together, however, fragmentation still occurs. File fragmentation can lead to slower systems because the hard disk heads must move around more to access the complete file.
The solution to fragmentation is defragmentation, which is the process of reorganising a disk so that files are contiguous.
Note: certain files will not be moved during defragmentation because they are system files. These file must remain where they are on the hard disk so that the operating system can find them.