These are two kinds of foods normally served at a dinner table TOGETHER. Yorkshire Pudding is made of eggs, flour, milk and water. It's not actual pudding, but yet similar to a muffin. Roast beef is a type of meat.

They are two different dishes. They are often thought of as "the" traditional English dinner. In the United States, where I live, "roast beef" is a very common dinner entrée, but "Yorkshire pudding" is not. "Pudding" is a word whose meaning is different in the U.K. and the U.S. The best way I can describe it is that in the U.S. "pudding" normally means something you could not possibly eat with a fork; it usually a thick near-liquid like yogurt or oatmeal. In the UK it seems to be a wide range of cake-like foods that are "moist" but not liquid.

These are yorkshire puddings: https://static-secure.guim.co.uk/sys-images/Guardian/About/General/2010/3/8/1268072179512/Three-Yorkshire-puddings--001.jpg This is roast beef: http://medinascatering.com/wp-content/uploads/2012/02/roast_beef-6574.jpg It's common to see these served together, but it's not always the case. You can serve yorkshire puddings with other kinds of roasted meat as well.