Aside from live performance, recorded films of dance are possibly the most informative sources for becoming familiar with various dance styles and aesthetics. Remember that these are usually highly edited and produced and may not be representative of what happened on stage.
An excellent guide for getting to know dance styles through online resources is:
The Media Collection at Douglass Library holds hundreds of recordings of dance that are available to students. Use QuickSearch to identify holdings in the collection. You can limit your search results to the resource type film/video and that it is held by the library. Here is a sample search.