Whenever you or a colleague changes an item, the change log is updated and a new revision is created. If you click on the history button on an item you can see the history dialog. 

A dialog will be shown with all versions on a time line. You can select one or more version to see the

• version number,
• the action (like add, edit, delete, undelete, etc.),
• the name of the user who changed it,
• the date and time of the change,
• the change comment

If you have two or more selected the differences between the selected versions will be indicated, e.g. below between version 3 change the font of a sentence compared to version 2.

Version 4 changed a whole paragraph. 

Look at the following video (direct link here) to see how to compare different versions of an item:

Note that, except for the last version, all versions have a RESTORE link. You can use this link to restore an older version of an item, thus replacing the last one with that older one.