how to chose TVS diode?

I was wondering if anyone knew how to choose a TVS diode.
They have spec'd "reverse standoff voltage", "breakdown voltage" and "direction" (uni or bi directional).

If I have a 5V Vcc do i want "reverse standoff voltage" or "breakdown voltage" above 5V?
Why would I ever want a bidirectional diode? that sounds like a horrible idea.

The reverse standoff voltage is the normal operating voltage which in your case would be 5V.

The breakdown voltage is self explanatory.

Bidirectional diodes are used to protect AC circuits.