Can I get my saved data back later if I uninstall a steam game?

This depends on the game.

• Some games store saved into the cloud by default. These will always be available upon reinstall.
• Some games store saves locally, but do not delete them when uninstalling the game. Either the saves remain in the game folder, or they are stored somewhere else (e.g. My Documents). Upon reinstalling on the same computer, the saves will be available again. If you want to install the game on another computer, you'll have to copy over the saves (if this works for the specific game).
• Some games could possibly remove all their files upon uninstallation; but I haven't personally encountered this yet.

This depends on the game. Some games uses Steam Cloud Sync to save the save. You can check out this question for more information about Steam Sync.