`diff` compares the contents of two files line by line. It can also compare directories. By default, `diff` shows you all the lines that are different between the two files. You can use options to get more specific output.
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Examples
Compare two files: diff file1 file2
Compare all files in two directories: diff -r directory1 directory2
Ignore trailing white space and blank lines: diff -bB file1 file2
Output file differences in context: diff -c file1 file2
Output file differences in context, specify lines of context: diff -C 3 file1 file2
Ignore case differences in file contents: diff -i file1 file2
Ignore all white space: diff -w file1 file2
Report only when files are different: diff -s file1 file2