A beta... is a progammer who tests computer codes and interfaces in the last stages of development.
You mean a fic
I'm an editor--a pro beta, if you will. But there are many different kinds of editor, and just so, different betas serve different functions. (Frequently, one editor will handle one function but never all--every text needs multiple sets of eyes.)
Here's how the editorial process works with books:
First there's aquisition--the book gets purchased. This is handled by... surprise! the aquisition editor. This function is handled by community mods. Then the manuscript is prepared--depending on how much work it needs this is called either developmental (that is, deep) or line editing--which involves moving the text around, making sure that there are no inconsistencies or discontinuities, that the prose sparkles and that the story or argument works. This is the kind of editing work I do best, and my favorite kind of beta. :grins at aberforths_rug
Then you go through the copy-edit phase, in which the style is cleaned up--grammatical, punctuation and spelling errors get caught and cleaned up. Most good betas do a great job of this. (I'm okay at this.)
Then the edited text gets sent off to the folks who typeset the book--this is automated in most fic situations, though it's occasionally handled by site mods--when they talk about coding, this is what they're talking about.
Then it goes through proofreading--an exhaustive final edit to make sure that there aren't any spelling, punctuation or gross style errors and that the typesetting hasn't introduced any new problems. Though this stage of development doesn't usually show up here in fic betaing, the proofreader is the most typical good
beta. Misspelling? All over it. Comma instead of a period? Taken care of. (I'm okay at this, but not great--more a big-picture person than a detail person, you know, and proofreaders are the epitome of, erm, anal.)
There are a lot of betas--if the thank-yous on fics are any indication--who see their role simply as being excited readers/fangirls. "ZOMG!!!!11!! SO KEWL!"
That's not a beta. That's someone doing the author a disservice by withholding constructive feedback.