Okay, so what is cabaret exactly? Well, cabaret is a lot of things. At it's core it is a show that mixes songs of the performers choosing, with spoken sectioned know as patter. The songs and the patter serve to take the audience on a journey. Cabaret tends to also be a more intimate event, even in the few large, Broadway style cabaret houses, because the best artists of the style make it feel like they are having a conversation with the audience.
Past that it really is a wide open field. Cabaret shows, can use musical theatre songs, pop music, jazz, rock, comedy numbers, sometimes all in the same show. Pretty much anything goes as long as the perform connects to it and the show as a whole has some sort of emotional journey. To help illustrate this idea I am going to post 3 different youtube videos. They are from various cabaret shows, they are all very different, but all 3 fit under the umbrella of cabaret.