When an extension is uploaded to TER, the developer should specify a comment. As usual, respect your users and provide a good comment that tells the users what the changes were. It is not worth pasting the full change log if it is too long (you have it in the ChangeLog file already). But outlining the most important changes is always a good thing to do.
TYPO3 follows the PHP versioning scheme. This means that there are three version numbers. The first number changes when something major is introduced, or the product is remade completely. When this happens, the remaining numbers are set to zero.
The second number changes when a certain feature is introduced, or a major bug is fixed, or lots of bugs are fixed.
The third number...