What is a fragmented file in a computer?

Files are stored on a computer hard drive in sectors. If a single file is stored in multiple areas (sectors) then it is considered fragmented. Basically, fragmentation slows down the computers abilty to to access a file quickly and efficently. A defrag program might reconnect the segments of the file and speed up the process of reading a file into memory. This is a simple explanation. For more information do a search for "defrag"