I think you could argue that intelligent design is a theory since it attempts to describe the mechanism for the diversity of living things. However, intelligent design depends on the existence of an external, directing entity for the changes (i.e. evolution) of life on earth. Natural selection on the other hand is self contained, requiring no external components, and therefore is generally regarded as the more robust theory of the two.
It is worth introducing you to Occam's razor, if you are not already familiar with this concept. In summary, Occam's razor says that if you have two or more explanations for how a natural mechanism works, the least complex is more likely to have occurred. A good example of this is the Heliocentric view of the solar system, versus the Earth-centric. In earlier times, it was believed that the Earth was the centre of the Universe. However, there were some observations of planetary motion which contradicted this view - essentially that some planets seemed to slow down in their progress across the heavens, reverse for a short while before continuing back on their way as normal. Various complex workarounds were produced to explain this phenomenon whilst maintaining the Earth's position as central to all other celestial bodies.
Placing the sun at the centre of the solar system with all the planets (Earth included) orbiting explained the observed phenomenon and removed the need for any complex planetary dances.
So, although I would personally say that intelligent design is a theory, it's not as good at describing evolution as natural selection since it is not self contained - i.e. who/what is the intelligence doing the designing and where did that intelligence come from/evolve/etc?
If you want some more background to flesh this out, go to the Wikipedia.