This question popped up over at Skepchick, and I thought it might be useful to know what my operating definitions were whenever I use these terms.
Here they are:
What is creationism?
Creationism is a related set of ideas concerning the origins of the universe, and humanity's place in it, and humanity's relationship with its creator. Creationism has two very distinct usages stemming from this definition: the political (using the capacity of the state to coerce curricula) and the theological (emphasizing the relationship humanity has with a creator). The theological is the less common in usage, especially in debate with regards to science and the incursions science has made in resolving (literally interpreted) mythical matters on the origins of the universe.
What is intelligent design?
Intelligent design is a creationist pseudoscientific political movement which seeks to displace actual science from western culture (per the Wedge Strategy). It has no other coherent definition.
What is theistic evolution?
A type of creationism that is largely apolitical; recognizing that theology is an inadequate tool for exploring the universe, it instead focuses on theologic aspects of humanity, religion, and the relationship humanity has with its creator. Though there are many variations within this category, the basis remains the same: teleological explanations are only acceptable where they do not conflict with science itself (and some would say, therefore, with creation itself).
It is perhaps not surprising that many scientists who are religious subscribe to this variant. It is also probable that it is, in fact, the most common variant (at least in the US.)