Theism
1. belief in God: belief that one God created and rules humans and the world, not necessarily accompanied by belief in divine revelation such as through the Bible
2. belief in god or gods: belief in the existence of a god or gods
[Late 17th century. Formed from Greek theos (see theo-).]
Atheism:
The prefix a- means "without" or "no".
Theism means "belief in god".
Thus, we have the final, literal and correct definition of the word; "without or no belief in god(s)".
This is how atheism is defined correctly. You will find that all major atheist organizations (American Atheists, Freedom From Religion Foundation, etc) acknowledge that literal definition as well. To molest the word by pulling some wacked usage of the word out of some websters dictionary and claim it to be the "real meaning" is ignorant.
An atheist is just someone who's without belief in gods - they don't have to make an assumption: "there is no god" to be an atheist.
Agnosticism:
This is a belief that something is unknown or unknowable. It doesn't necessarily have to do with a deity, but of course in this context it does. Agnosticism isn't a "3rd choice". It is *not* a middle ground of any kind.
People must be either theist or atheist, one or the other.
There is no other alternative. You either believe in something or you do not. You're either theist or you're atheist. If you claim that you are "undecided", well, that means you're an atheist. As long as you don't assert there is a god, you are an atheist. If you claim you're an agnostic, you must still either be theist or atheist. You can be an agnostic-atheist, where you say that gods are unknown or unknowable, so you remain without belief - or you can be a "full of faith" llama and be an agnostic-theist, where you say that you realize that gods are unknown or unknowable, but you're going to "believe anyhow" using "faith" or some bullshit cop-out like that.
Christianity
1. religion that follows Jesus Christ’s teachings: the religion based on the life, teachings, and example of Jesus Christ
2. holding Christian beliefs: the fact of holding Christian beliefs or being a Christian
3. Christians as a group: all Christian people considered as a group.
See also Christendom
Catholicism
Catholic beliefs: the beliefs, doctrines, and rituals of a Catholic church, especially those of the Roman Catholic Church
2. membership of Catholic Church: membership of a Catholic church, especially of the Roman Catholic Church
Protestant
member of church rejecting papal authority: a member or adherent of any denomination of the Western Christian church that rejects papal authority and some fundamental Roman Catholic doctrines, and believes in justification by faith. The formulation of Protestants’ beliefs began with the Reformation in the 16th century.
Just to name a few i will add more at a later time ~Yoda