If you running 64-bit system and want to run steam games (32-bit) you need to install 32-libs to your system.
You can find it in Ubuntu Software Center: ia32-libs
or the faster way via terminal:
EDIT: terraria is Windows only game and do not run natively under Ubuntu. You may try wine and through that install Windows Steam, but it's a hit'n'miss when it comes to Windows game on Linux. Better to dual boot or use games that are available for Linux.
sudo apt-get install ia32-libs
I'm moving this thread to the wine section.
You may try take a look at the stickies in our wine section, lot of good info on the subject.