There are tons of "classifications" and most of them are more in the eye of the beholder than anything. Getting overly wrapped up in those kinds of definitions will drive you nuts.

"scale" usually means that the model's appearance is based on a "full scale" piloted (usually) aircraft. How accurate the representation is varies a LOT, and the word "scale" does not imply how much it does or doesn't at all. (in other words, I disagree with some of the posts above). "scale" is more in the eye of the beholder, and what is and isn't "scale enough" for their tastes.

"sport" usually means a model that is NOT based on a larger aircraft. Most trainers and planes like the 4-star would fall into this category.

There are dozens more that you might see:
warbird - a scale model of a military aircraft, almost always a particular individual aircraft.
giant scale - any model (scale or sport) with a wingspan over 80" (for monoplanes, 60something for biplanes, if I recall)
pattern - a model designed to fly the AMA or FAI acrobatic precision routines "patterns".
IMAC or scale acrobat - models that are designed for IMAC acrobatic routines (kinda like, but different from pattern). They are very approximately scale models of piloted aircraft with totally non-scale color schemes.
3D - models designed for a particular style of acrobatics that are mostly performend at slow speed and high angle of attack.
profile - a model that looks like the body was run over by a steam roller

And lots more, and combinations of the above in some cases.