If /home is on a separate partition, then you shouldn't have anything to worry about; if it's on the same partition with other things, though (like / or /usr, which will be replaced with a reinstall) then it might get blown away. One way to find out, if you don't know, is to look at /etc/fstab. If there is a line something like:
/dev/hda5 /home ext3 defaults 0 0
then your /home is probably on its own partition. Another way to check is to type 'mount' from a terminal window. If there is a line for /home in there, then it's on a separate partition. As long as you don't re-format that partition, a reinstall won't affect it.
If, however, /home is part of the / partition, you may want to create a spare partition, or use an existing extra partition, to backup everything to it, and then use that partition for /home after you reinstall. It's a good idea to keep it separate, so reinstalls won't affect it.