kleggy:
I know while doing automotive electrical you can get a relay that powers high powered spotlights with minimal strain on the switch itself. Is there any such thing for LEDs??
You could similarly use a relay but that's a bit overkill for LEDs and the constant clicky clicky of the relay would wear it out in a hurry. You would want to look into driving a mosfet with the Arduino as the mosfet would serve the purpose similarly but in a solid-state way.
The trick with LEDs is that you need to control the current they consume. As is typical for any diode -- which a LED is -- after you pass the threshold voltage every additional bit of voltage causes the LED to rapidly consume more current until the point that they burn out. This occurs within the span of just a few tenths of a volt, and when you're dealing with a bag of 100 LEDs you're never guaranteed that they'll all have perfectly identical threshold voltages. Thus, pulse-width modulation (PWM) with a transistor, mosfet, or an IC like the TLC5940 is used to rapidly chop the current to the LED to prevent the burnout.