In the Buffyverse, anyone who says the right words in the right order can use magic, but some people are much more skilled in it than others. There's also a strong cultural belief among witches that overusing abilities = baaaaad.

Tara has a great deal of potential, but right now what she will mostly use it for is telekinesis, charms (finding lost objects, for example) and glamours. Anything heavier than that -- conjuring -- will freak her out unless there's a really good reason to use it.

She also sees auras. Canon's vague like a vague thing about what this means; I would say she can tell general state of mind (OOCly, what's in narration) and if someone is possessed or has something else obviously wrong, but not general supernatural status.