For the First Time
Thomas Henry Huxley is believed to have first coined the concept of agnosticism, and thus came the term agnostic.
Before we begin with this article, it is very important to understand what God means. This term is commonly used by everyone whether that person is a firm believer, an atheist or an agnostic. The difference between these three categories of people lies in their interpretation and belief regarding the existence of "God". For some, God means a supreme being or a deity, who is the creator of this world, for others there is no proof of such supernatural existence, so he possibly couldn't have created this world, then there is another category which oscillates in between these two categories.

They are those who can't really understand which category they should pick in order to be in the right sense of the mind. It wouldn't be wrong to categorize them as victims of confusion. So, which category do you belong to? Well, for your aid you have this Buzzle post which would be able to guide you in choosing your category.

This article aims to clear the differences (both subtle and deep) between the concepts of an atheist and agnostic. However, before going into the differences, let us know for a better understanding, the definitions of a theist, atheist, and agnostic.

A theist is a person who believes in theism. Theism is the faith and belief in existence of deities or Gods. A believer is a person who is theist or in other words, believes in the existence of God as the supreme being and the creator of this world. They have unconditional and unquestionable faith in God and his capabilities.

It is in God, and him alone that the theists rest ultimate faith. He is worshipped as the ultimate truth, and he alone is attributed for everything that the past, the present, and the future hold. Thence came into existence several faiths and their respective followers.

Atheism is the absolute disbelief in God. An atheist is a person who doesn't believe in any kind of supreme being. Mainly because they feel that there is no proof of such existence, moreover science can't vouch for the same.

Atheist don't have blind faith in any deity. They believe that life is a process where there is no room for existence of a supernatural being. This belief also stems from the question they put up regarding the presence of evil in the world. Atheist believe if there is God, there would be no evil on earth. They firmly ask, "if there is God, would he allow such evil to cultivate on this planet?"

Agnostics, on the other hand, are neither believers or non-believers. This group has doubts over the existence of God. They can't for sure say that they believe or disbelieve in the existence of God.

But they choose to live their lives as governed by certain principles, beliefs and morals. Agnostics also believe that one can never determine whether God exists or not, so they choose to live their lives according to a set of beliefs irrespective of the knowledge or belief regarding God's presence in the world.

They feel that knowing whether God exists or not is not of much importance. Agnostics are open to the idea of God and leave room for the doubt that if there is God, he has his mysterious ways to work.

Agnostic Vs. Atheist
Agnostic is the one who neither believes nor disbelieves in God.
Atheist is the one who rejects the idea about the existence of God.
Origin can be traced back to 1869, with the first use in 1870. It originated from the word, agnostos, which means unknown, or something about which nothing can be known.
The term originated somewhere in the year 1570, from the French word, athéiste, the Greek word, atheos which means denying god, godless.
Key Proponents
Susan B. Anthony, Thomas Jefferson, and Carl Sagan etc.
Christopher Hitchens, Dan Dennett, and Sam Harris etc.
Governing Philosophy
The guiding principle of this school of thought is knowledge. The agnostics believe that since there is neither of the evidences which confirm the presence of god, nor his overall absence altogether, therefore, the knowledge is inadequate and hence they are neither theists nor atheists. However, they don't rule out the possibility of affirmation of the presence of god, some day in the future.
Their logic states that if there is evil spread out in the world, then there can't be the existence of God. They say that, how the Creator could let such widespread evil destroy his creation. Why are there so much of divisions amongst his own people who fight and kill in the name of god and religion. Why are there sects in a religion, why are the masses discriminated on the basis of creed and class.
The two types of agnostics are empirical agnostics, who believe that there are chances of God's existence and things are more or less controlled or governed by this supreme being. Another type is agnostic humanist, who on the other hand believes that the particular question of existence or non existence of God is not of much importance. They in any case, lead their lives according to a set of secular beliefs. These beliefs are more in line with "live and let live". Charles Darwin,Thomas H. Huxley, and Francois M. Arouet also fall into this category. M. Arouet, popularly known as Voltaire, who was a French author is considered as the father of agnosticism.
A weak atheist is the one who lacks faith in god. He is also known as an Implicit atheist. A strong atheist or an explicit atheists is the one who not only expels the belief in divinity, but also rejects the idea of divinity. A broad atheist is the one who doesn't acknowledge the presence of any gods, or deities, or demi-gods. A narrow atheist on the other hand expels the concept of god's power as all mighty and all powerful. Ayaan Hirsi, Douglas Adams, Dave Barry are some more atheists.

Everyone has a different take on the atheist vs. agnostic issue, for some they are pretty similar whereas for others, atheist and agnostics are two sides of the same coin. It doesn't matter which side you belong to as long as one is tolerant towards everyone's beliefs. To reach the goal of peace and prosperity, eventually we have to learn to live in harmony with each other.