It kind of depends actually ( ´ ▽ ` )ﾉ. Like if you're reading an aboverse/omegaverse: omega is the male that gets pregnant, the submissive one; alpha is the dominant one who is able of impreganting the omegas, and beta is the alpha's subordinate, it may or may not get an omega pregnant and even a beta can take the omega's role, depending on the author. If you're reading a werewolf-like au: alpha, beta, and omega are like hierarchies. Alpha is commonly the leader of a pack and it is on the highest hierarchy ranking, beta is some sort of right-hand to the alpha, and omega is the lowest ranking of all.
Wolf!au's will almost always contain knotting, and in general when a pairing has in a wolf!au they become mates.
Hope this helped! ヾ(｡･ω･｡)

Hmm it depends on the author. But the norms are, the Alphas are the leaders while the Betas are their underlings. Omegas are usually those who can get pregnant, and one that Alphas mate with. I often read mate coming from their scent or something, as a form of attraction. I am not quite sure about the whole wolf transforming, but those are usually up to the author's imagination. We can do anything with wolf!au, really lmao. Oh yeah, there's the full moon and it usually relates with Alphas in rut or Omegas in heat. That's like, their calling to mate (shecks!). Ahhh, about the knotting and mpreg, the mpreg is not always from wolf!au. Knotting, yes, wolf!au always come with knotting. ;)