People cheat, it's in our DNA. Whether we want to admit it or not, cheating is an inescapable part of the human condition. Some of us are more comfortable than others acknowledging this fact, but it's still a fact, plain and simple.
Cheating is anytime you would not want your partner seeing you do. We cheat on our partners for all kinds of reasons—it has nothing to do with them. We cheat because we’re pissed off, we cheat because we’re insecure, we cheat because we’re lonely, we cheat because we feel like it. We have probably cheated on every single partner that we have been with. Maybe we haven’t had sex with people outside our relationships (or maybe we have), but we’ve had those gut-clenchy moments of, I can’t tell my partner about this.
In the word of Cameron Diaz: "We are human beings, we are complicated. You cannot go through life without tallying up a few scars, you cannot go through life unscathed, it's just what it is. It's all meant to happen, take your lessons, figure it out, move on."