i.e. is a circuit with resistors much less efficient than having the correct voltage/current from the course? i'm thinking here mainly about dimmer switches and also parallel LEDs where each would need its own resistor. Cheers.?

A resistor will generate heat whenever a current flows through it, so the more voltage you're dropping across it, the more power it will waste as heat. The formula is P= I^{2} x R (the current squared times the resistance) or V x I (the voltage across it times the current through it).

also remember that, as well as parallel, you could put some LED's in series. depending on your source, put enough LED's in series to sink almost that much voltage, then put groups of those series LED's in parallel. you'd only need one resistor (for current management as frollard says, for the safety of the LED's) in series with each group. less resistance, less waste. here's an illustration i whipped up in case i suck at explaining.

ideally you want the voltage source very close to the led source - so you only need a very small resistor. Even when they're matched, you still need a resistor (say 1 ohm) to make sure that small fluctuations in manufacture of the leds mean each one draws the same current. as Andy says - the heat dissipated is the square of the current - so you want the right current, or a small fluctuation will cook you.

active| newest | oldest^{2}x R (the current squared times the resistance) or V x I (the voltage across it times the current through it).