I really want to create this effect with two coloured lighting. I read this up but I was confused about the gel lighting. Can someone put this effect into simple terms? Thanks in advance.
"Gelling" a light simply means putting a sheet of colored plastic (a gel) in front of the light so the light is tinted. In this case, the light source on one side of the subject's face is purple and the other side is blue, so purple and blue gels were used on the two light sources.
With Strobist type setups, gels are fixed to the front of the flash head in all sorts of ways: holders, velcro, etc. But you do want to be careful not to melt the thin sheet of colored plastic by overheating it.
The other thing you have to be very careful of, when setting up this kind of lighting scheme is to avoid combining the light sources into each other, or the colors will mix (in this case, you'd get blue, purple, and then a purple-blue area), so flagging off (putting blockers on the sides of the lights) the light sources, or using grids can help control spill and narrow the light sources. And you usually have to kill the ambient light (i.e., make sure your ambient/flash balance is all-flash and no ambient--typically, using exposure settings that, without flash, would underexpose by four to six stops).
If you plan on coloring a background, using a black backdrop will create a more saturated color than using a grey or white one.