SPDT Switches have three screws. Power comes IN on one, and out on ONE of the other two. (or, conversely, in on one of the two, and out of the single-screw.)

DPST switches are like two regular SPST switches stuck together sideways, so that both are either OPEN or CLOSED at the same time. I've never seen a situation where that was useful.

DPDT switches in the context of multiple swtiches controlling one set of lights are wired so that with the switch one way, the two incoming wires (red and black) feed strait through to the same color outgoing wire, and with the switch the other way, the red incoming crosses over to black, and the black incoming crossed over to red. A "Normal" DPDT switch has 6 connector screws on the back. The two incoming screws in the middle each connect to a pair of OUT screws, one of which is UP, and the other of which is DOWN from it. You have to connect the UP on the left and the DOWN on the right to one outgoing wire, and the UP on the right and the DOWN on the left to the other outgoing wire. *BUT*. Leviton makes a "DPDT" switch that accomplishes this Straight-through / Crossover business internally, and provides you with only 4 connector screws, 2 in, and 2 out. This makes it LOOK like a DPST switch, so you have to pay attention, but it does make a much less tangled switch-box.

You should only ever have one wire under any given screw. If you need/want more than one, what you do is get a short, third bit of wire, and connect THAT to the screw instead, and then connect that wire and the two or more other wires together with a wire-nut. Usually this is only an issue with the ground-wires.

Most switches have a green-tinted screw somewhere on them. This is for the ground-wire. Some wiring boxes ALSO have a ground-screw attached to them. Hook all the ground wires together with a wire-nut.