JoVE is an interesting resource - its a video platform, but it is also considered a journal. Consult your preferred style guide and cite this resource like you would an article in an electronic journal.
For example:
Welstead, G. G., Brambrink, T., Jaenisch, R. Generating iPS Cells from MEFS through Forced Expression of Sox-2, Oct-4, c-Myc, and Klf4. J. Vis. Exp. (14), e734, doi:10.3791/734 (2008).