Steam allows you to choose which games you keep up to date, but all games are set to update automatically by default. If you're on the beta channel, you can give high priority to certain downloads.
If you're not already part of the beta, here's how to join:
- Under Settings in Steam, click the Account tab.
- Click "Change" below "Beta participation".
- Select "Steam Beta Update" to enter the program.
- Restart Steam.
Once you're in the beta, here's how to prioritise certain downloads:
- Right-click the game you want to prioritise in your library and select "Properties".
- Select the "Updates" tab.
- In the "Automatic Updates" drop down, select "High Priority".
Steam excels at managing a huge library of games, but if you have a lot of titles installed, you can end up with an update for your favourite game waiting in line behind a queue of 12 things you got on a Humble Bundle. This solves that problem.