On this video of a recipe for mushroom-onion pudding, I can't seem to recognize which mushrooms are used (at about 2:20 in the video).
What is the name of these big mushrooms?
I'll elaborate on Joe's answer some. It's a little known fact that button mushrooms (also called common, white, or table mushrooms), cremini (crimini, baby portobello, baby bella, brown mushroom), and portbello (portabella) are all the same species. Specifically, agaricus bisporus. The difference in appearance and taste is based solely on maturity. The youngest are white button mushrooms, and the oldest are the big portobello mushrooms. There can be slight differences in skin color for the portobello's, though it tends to be darker than the stark white of the button.
The mushrooms in that video are most certainly fully mature agaricus bisporus, a portobello.
It's a safe bet that anytime you see a mushroom being substituted for a "steak", that it will be a portobello. They have a rich meaty flavor and texture that is perfectly suited for this.
This is sort of a trick question. Technically, it's a crimini mushroom that's been left to mature before harvest. Due to marketing, they're now sold as portabello (or portabella), and the smaller criminis are sometimes sold as 'baby bella' or 'mini portabello'.
update : I should've mentioned; that's what they're called in the US. I believe 'portabello' is used in the UK, based on watching too many cooking shows, but I have no idea if they might go by any other names, or if those names are used in all countries.