What is the difference between a "trading security" and "available for sale" investment?

In our accounting chapter for Financial Instruments and Liablities, I am having a lot of trouble knowing the difference between a trading security and security "Available for Sale". Also, if a bond is sold before maturity even though it was originally intended to be held to matrurity, is it then treated at a trading security. And.... which ones go under OCI and which ones go under Income Statement?