MD5 is a commonly used check-sum designed to warn if a file has been edited or become corrupted. Given an input file (or piece of text) the MD5 algorithm generates a 128-bit sequence (typically represented as 32 hexadecimal digits). For instance the MD5 value of the text

"The quick brown fox jumps over the lazy dog"

is

"9e107d9d372bb6826bd81d3542a419d6"

Changing a single character in the text (or input file) results in a very different MD5. Hence by calculating the MD5 for a file and comparing it with a previously calculated value for that file, it is (almost always) possible to tell whether the file has been changed in the interim.