Use this online free Code Diff Tool for comparing two text files.
This tool provides an easy way to highlight the differences between the two inputted texts. Using the tool is super easy; input the two texts in separate boxes and you can see the output right below. It will graphically show you the differences between the 2 text areas by highlighting those changed areas in red color. You can choose whether you want to see the difference between two given texts by characters, words, or lines.
The diff utility is a data comparison tool that calculates and displays the differences between two files. It displays the changes made in a standard format, such that both humans and machines can understand the changes and apply them: given one file and the changes, the other file can be created. It is used to show the changes between two versions of the same file. Modern implementations also support binary files. The output is called a "diff", or a patch since the output can be applied with the Unix program patch.