I asked the other day what is the definition of "cheesing" other than the sneaky things you do to your opponent that they don't like. I think I found out one of the ways of making a distinction between smart tactics and the sneaky tricks of the weak.
Cannon rushing is one of the "cheesy" tactics that Protoss can use. Photon Cannons are a structure which you are supposed to use for defense. It is as the name implies a canon which gives you some pretty solid defense against both air and ground units. But it can be used as an offensive weapon well, by placing a pylon in your opponents base and then setting up canons around to harass and give them a headache. There is a science to where you place your pylons. You can block off their ramp, try and put it directly in their main base.
A cannon rush can be a legitimate tactic and not always a weapon of the tactically weak. After watching it used in the GSL a couple times, used against my brother and also see others use it online I think that to rush it onto the main or even blocking their ramp is a tool of the desperate. It is an attempt to kill the other before you or them can even start the match. When these tactics are used against better players they tend to fail miserably. Because if they fail, you wasted a significant amount of resources with usually little to show.