Every beta reader will work differently. Most will provide help with grammar and spelling, although you really should spell check your own work before submitting it to a beta reader. Beta readers are also generally familiar with the fandom as well, and can assist with questions about the canon setting and characters, as well as offer guidance about the direction of your story.
So, to some extent, beta readers are like editors, with one distinction. Most beta readers are not professionals, while most people who can rightfully call themselves an editor are much more likely to be fully versed in the grammar, punctuation, and style requirements for a publisher, as opposed to a fan fiction archive.
My recommendation to you is to look for a beta reader who is familiar with your fandom, read some samples of work they have beta'd, and discuss how you work and what your expectations are. A beta might have a real life situation come up which will delay them in working on your story. How flexible are you? Some beta readers will not take work that isn't at least spell checked and reasonably formatted. If you don't want to prescreen your work, you may lose out on the services of better beta readers who don't want to waste time performing spell checks for you. Do you want them to make changes for you, or just give you a nudge in the right direction? You should talk all this out beforehand, so there are no surprises.
