Port trunking involves uplinking switches together using multiple ports, at their maximum speed. Basically this means you would could use two (or three, or four, or more, depending on switch) ports on each switch, trunked, for maximum throughput of 400 mbps (100 mbps + 100 mbps upstream, 100 mbps + 100 mbps downstream), again, assuming full duplex 100 mbps Ethernet.

Prioritizing ports is something different, but can be useful in certain situations. For example, you may want to give a dedicated game server high priority on a switch, so then the people doing file transfers can have higher latency, and slightly slowed transmission. This is really only going to have an effect if you're using extremely high amounts of bandwidth on the switch, though. Additionally, some switches allow manual specification of 10, 100, or auto-sensing speeds per port. You may want to restrict people to 10 mbps if there's game slow-down.