What exactly is plaque anyway?

Would Sir[1] prefer the long boring scientific answer or the tl;dr[2]?

The tl;dr it is then. It's the gunk created by mouth bacteria which has its malign effect because it prevents Oxygen reaching the bacteria in the lower levels. This forces the bacteria, starved of Oxygen, into anaerobic mode. Misery ensues.

[1] Other excellent genders are available. 

[2] Too long, didn't read. I'm down with the kids, me.