Questions about specs:
Does "collector current" mean the transistor can handle up to a 5 amps load?
(With enough heatsinking)
Does "base current" mean that the arduino needs to supply 120 mA to trigger the transistor to "flip the switch?"
No. Numbers under "ABSOLUTE MAXIMUM RATINGS" are the values that will kill the transistor.
The number you're interested in is under "Saturation voltage"
Does "Emitter-base voltage" mean that I need to put 5V on the base to get it to switch?
No. (see above...)
You need "Base-emitter on voltage" further down.
Questions about wiring:
To wire this up would I do this?
+12v supply -> collector
Arduino PWM pin -> base
+ side of LED wires -> Emitter
- side of LED wires -> ground
No. The transistor has
to go after the LEDs, with the emitter connected to ground.
You'll need a resistor between the Arduino pin and the base. Try 150 Ohm.