There is a free and open source software for this job called Melt Diff Viewer

Meld is a tool which allows the user to see the changes in, and merge between, either two files, two directories, or two files with a common ancestor.

You can install it the details and screen shots given below..

1. Go to Ubuntu Software Center

2. select "Get Software"

3. Type "meld" on search text field and press enter Then click on "Install" button to install it on your PC

You can install Meld using terminal with the command : sudo apt-get install meld

4. After the installation go Programming-> Meld Diff Viewer

5. Click on New button

6. Then Browse the path for files and click ok. Then you can compair as you need...

The End ...