If you come in two-footed, studs up and/or approach the tackle with excessive force I would say that's a dirty tackle.
There's nothing wrong with going in strong, hard and fair but sometimes it gets to a point where you wonder what was going on in the player's head.
Okay, here's what I am talking about. In the majority of tackles in this video, Rio Ferdinand stays on his feet and if he does go down, his studs aren't showing before he's even made contact with the ball, man or both. The first tackle on Joe Cole is pretty bad, though.
With that in mind, here are some of the dirtiest tacklers in the English Premier League past and present.