It depends on whether or not you have a separate /home partition, whether you use mysql and how much tinkering you have done.
If you have a separate /home partition, all your personal data will be there.
If you use mysql, you need to use mysqldump to backup your mysql databases to your /home partition.
If you tinker a lot, you probably also need to backup any files in /etc which you have modified to your /home partition.
Once you have everything you want to keep on your /home partition, back it up and proceed. Most distros will automatically leave /home untouched but it is better to be safe than sorry. (I have never had to use my backup.)
If you don't have a separate /home partition, you will need to back all these things up anyway. When you install the new version, look for an option to create a separate /home partition to make things easier in future.
(Note also that a handfull of other programs (e.g. evolution) will only restore, like mysql, from their own backup format. So back these up using their own format before you back up /home.)