Explaining diff output

$ diff file1 file2
3c3
< this is a line in a file1
---
> this is a line in a file2

Explanation:
3c3 - LinesFromFile1-Action-LinesFromFile2
Action - (a)dd (c)hange (d)elete
< - line from file1
--- - merely separate the lines of file1 and file2
> - line from file2