Lately, a lot of people have been referring to themselves as agnostics instead of religious (theist) or non-religious (atheist). This is wrong because agnosticism is an answer to an other question.
There are actually two independent discussions here. "theism vs atheism" and "gnosticism vs agnosticism". Although they have strong ties to eachother you can't mix them up, which is why I'll explain them here.
You believe there is a god.
You believe there is not a god.
You think that the truth can be (and is) known.
You think that some truths can not be (and aren't) known.
Theism is the belief in at least one god and the opposite of atheism. Theists usually have a religion that describes their god, and gives lots of contradicting rules and morals, but theists don't have to be "religious" and may simply believe that there's some kind of god or "higher power". This is often because they don't agree with the holy rules and morals of specific religions but still need to feel like there's a "greater purpose" in life and/or something that corrects for injustice. And also because of fear of death.
Atheism is the belief that there are no gods and the opposite of theism. It's still possible to have a "religion", such as forms of buddhism but then it's more like a philosophy. Atheists have usually accepted that there is no greater purpose and that justice is something we have to work on ourselves. Atheists are of course bound by evolution to "fear death" but often understand that all things eventually come to an end and that's that.
Gnosticism is the belief that truth is, or at least can be, known and the opposite of agnosticism. This often, but not always, refers to gods. Most theists are also gnostics. Since they're sure there is a god, they must also believe that the existence of that god can be known. Although they're usually agnostic about the mysterious ways in which this god works.
Agnosticism is the belief that truth isn't, or even can't be, known and the opposite of gnosticism. This, too, often refers to gods but doesn't have to. Most atheists are also agnostics because even though they believe there is no god, they also hold that we don't know if there is a god or not and some even say that we can never truly know.
Theists without descriptions of their "higher power", are agnostic in addition to their theism because they know that they don't know, and may also believe that they can't know, the truth about these higher powers.
Agnosticism is not a middle ground between theism and atheism but a separate question. If you're not sure if you believe in gods or not, you're not agnostic – you're just not sure. You're most likely the thing that you weren't before you were unsure.
You're agnostic if you think we don't, or can't, know if there are gods or not. Whether you believe in gods or not (theism or atheism) is an other question, although in that case, you're probably an atheist.