BJT Amplifier in AC mode
In linear circuits with both AC and DC components, one can split the circuit, using the Superposition Theorem, into a DC equivalent circuit to model the effect of the DC components and an AC equivalent circuit to model the effect of AC components. By "components" I mean voltage sources and current sources.

For AC analysis, you are only concerned with time-varying voltages and currents. Therefore, any DC voltage sources are considered shorted (0 V of voltage drop across them), while DC current sources are considered open circuits (0 A of current through them). Of course, the DC sources are included in DC analysis while all the AC sources are made 0. It is exactly the same as applying the Superposition Theorem to circuits with multiple DC sources - you consider the effect of one DC source at a time, and negate all the others.

This is the best link I found for explaining AC analysis so far.