An oratorio is like an opera - but less. There is singing and a storyline but not much else.
Oratorios have a lot of recitative.
Recitative is the part of operas that most people make fun of. Its where a person will sing-talk lines, usually on one note, and usually pretty rapidly. Its used to get a bunch of miscellaneous text to the audience. It is rarely beautiful, but it works. To understand why the English popularized the oratorio, we need to look at Handel.