Kink is an umbrella term used to describe a wide range of sexual activities that are considered to be unconventional or unorthodox. While BDSM is one of the first things that springs to mind when someone hears the word kink, it's far from the only kinky practice out there. Sexual acts such as spanking, dripping candle wax on the skin, participating in orgies, swinging and role-playing are also often considered to be kinky, as are some unusual sex positions or usage of sex toys.
Anything not kinky is said to be vanilla.