FicWad

A beta's like an editor. He/She looks over material and corrects things like spelling, grammar, punctuation, etc.

Like a live proofreader.

Except better. Automatic spell/grammar checks just change what doesn't seem right to the computer. A beta person can interpret the different ways an awry phrase might be construed and offer advice on how to clearer state the idea.

Also, unlike a computer program, and more like an editor, a beta can point out consistency flaws in a story. Like maybe in a certain chapter a writer somehow causes Harry to get perfect vision as the result of a defective curse and he discovers it when he notices his glasses are missing and he can still see. Later, the said author absentmindedly puts in something about Harry wearing his glasses. The beta would notice that detail and point it out or ask about it.

For example, I myself beta over at Fanfiction.net- I practically live on the Twilight section. Once in a while I'll have to point out to somebody that Emmett was attacked by a black bear, and not a grizzly, as there are no grizzlies in the Appalacians. Just little intricacies like that.