What is Steam trading?

Bit of a noob question but from reading the Steam pages on trading I don’t know what exactly can be traded. Can I trade any game? Can I only trade things I have never installed? Can I only trade things gifted to me? I just learned how to sell those silly cards you get from playing and have so far made $0.89! They say the first million is the hardest but I’m well on the way :)

Before somebody says it, I have no Steam friends to test trading with…

The OP asks not for donations of tissues to dry his tears from having no friends but just a simple answer to his question.

I believe you can trade games purchased as gifts not installed, as well as steam items like cards and team fortress stuff.