I'm not Italian, but I grew up in an area that has a significant Italian population. Guido is a first name, and I know people, named Guido. It's kind of like what Paddy is to the Irish, a name to identify a stereotpe.

Typically, I've always heard this term (mostly from other Italians, like my wife) to describe certain Italian (and non-Italian) males, who exhibit certain stereotypical traits, such as a swarthy complexion, thick black greased hair, jewelry, etc. My wife just used this term over Thanksgiving when she was describing my brother-in-law's "Guido shoes." I rarely hear the term used with viciousness or malice, but I avoid the word unless I'm joking with Italian friends. As olivant and pizzaboy pointed out, it can be offensive.

I thought it was odd to use the term in promoting a tv show, but I don't know what's going on with these kinds of words anymore.