What is bottleneck?

A bottleneck is when one part cannot fully utilise another part. Its most commonly seen with a GPU and CPU. To explain it better, lets say we pair a Pentium 4 @ 3 GHZ with a GTX 690. Thats a bottleneck, as the CPU processes information too slowly, which forces the GTX 690 to slow down, which causes less frame rates. A bottleneck could also be with a Power Supply, where that the amount of power a system is using is too little for the power supply to actually get its full efficiency. THERE will be NO bottleneck with a i5 3570k and a 660ti in SLI, that isn't even a bottleneck with two GTX 690's if you overclock the cpu a bit.

^+1
Everything he said is true.

Watch this video, it also explains a lot about bottlenecks.
http://www.youtube.com/watch?v=TGdo75gasaQ

Brilliant, thank you so much, really cleared things up for me!

short version ..... the weakest link.