Intelligent Design is the belief that some non-defined "intelligent agency" created life on Earth, rather than it all having evolved.
Bizarrely, many Cdesign proponentsists claim that ID has nothing to do with Creationism or religion, when even one of the founders of the ID movement explicitly says its purpose is to get people thinking about Jesus.
ID's main "arguments" are to try to poke holes in evolution, under the presumption that if current evolutionary theory is wrong, then ID must be right. See also: false dichotomy. Most of the objections ID raises are at best valid critiques, and at worst, flat out wrong and deceptive.
The main weakness of ID is its central argument, that life is too complex to have evolved on its own, so it must have been created by an Intelligent Designer. Of course, they conveniently ignore the fact that this Designer must itself be rather complex, so it would need to have been created as well, ad infinitum.