What does aperture and the mm in camera lenses mean?

I just purchased a Canon Rebel T1i and am loving it. I kind of understand the aperture (the lower the F number, the more sharp the object at the correct focal length, and the more blurred the background). However, I really don't understand what the 18-50mm and 55-250mm designations in my lenses mean. I think lower numbers are wide angle and higher are telephoto. Are there set limits, or is it relative to the range of your lens? Anyone care to explain?