So glad to see maybe you're de-lurking? :) I'm always hopeful because being active in fandom is so much fun. ;)

Well, I'm no expert or an authority in the matter, but as I understand it, beta reader is someone who helps a writer giving them feedback, helping them with the grammar and spelling, as well as giving suggestions/opinion (if the author indeed wants them) about characterisation and plot, consistency, etc. Sometimes it's not only about big edits, but smaller things, though just as important, like making sure a sentence reads correctly, you know?

They (betas) sort of go with a fine-toothed comb through the story making sure all those little details are sorted and the story is ready for posting.

An alpha reader does a lot of that, too. But they are like, I don't know, like the first line of defence. :D I kind of think an alpha focuses more on feedback, on telling the author what's working, or if a scene needs a little more work, a detail explained better, checking for consistency in the plot/characterisation, and of course, if an alpha catches a spelling/grammar mistake, is also there to point it out.

It might seem that an alpha and a beta do a lot of the same things, and maybe they do, but I think the big difference is to the author, because they've had two people reading their story, so they have a better idea what worked and what parts needed to a bit more of work.

Another thing is, after a story was beta read, the author went through their beta's notes and made some suggested edits, it means that story is ready to be published, you know? Not everybody needs or indeed "uses" an alpha, but a beta reader, is definitely someone you need, either to check grammar or spelling or if you [also] need a more in-depth feedback, a beta can be of great help with all those aspects.

Mostly people who uses alpha readers (I say uses, but the truth is, often alpha readers are like an author's confidant, who often times held the author's hand through the entire process of writing a story) also need a beta. But it's very common, maybe even more for people to only have beta readers, sometimes more than one, because fandom is about giving. ^^

I have no idea if what makes sense. Maybe one of these days I could ask my friends list what they feel is the difference between alpha and beta readers. :D