There are several ways to think about the differences between two files.
One way to think of the differences is as a series of lines that were
deleted from, inserted in, or changed in one file to produce the other
file. diff compares two files line by line, finds groups of
lines that differ, and reports each group of differing lines. It can
report the differing lines in several formats, which have different
purposes.
GNU diff can show whether files are different
without detailing the differences. It also provides ways to suppress
certain kinds of differences that are not important to you. Most
commonly, such differences are changes in the amount of white space
between words or lines. diff also provides ways to suppress
differences in alphabetic case or in lines that match a regular
expression that you provide. These options can accumulate; for
example, you can ignore changes in both white space and alphabetic
case.
Another way to think of the differences between two files is as a
sequence of pairs of bytes that can be either identical or
different. cmp reports the differences between two files
byte by byte, instead of line by line. As a result, it is often
more useful than diff for comparing binary files. For text
files, cmp is useful mainly when you want to know only whether
two files are identical, or whether one file is a prefix of the other.
To illustrate the effect that considering changes byte by byte
can have compared with considering them line by line, think of what
happens if a single newline character is added to the beginning of a
file. If that file is then compared with an otherwise identical file
that lacks the newline at the beginning, diff will report that a
blank line has been added to the file, while cmp will report that
almost every byte of the two files differs.
diff3 normally compares three input files line by line, finds
groups of lines that differ, and reports each group of differing lines.
Its output is designed to make it easy to inspect two different sets of
changes to the same file.
These commands compare input files without necessarily reading them.
For example, if diff is asked simply to report whether two
files differ, and it discovers that the files have different sizes, it
need not read them to do its job.