Atheism is the non-belief in deities. Put differently, it is the lack of conviction in the existence of any number of gods. Atheism is NOT the lack of religion(areligion, irreligion), the negation to religion(anti-religion), or a religion itself. While many atheists tend towards naturalism/materialism, skepticism, science, anti-theism, etc., many do not. Many atheists are religious, many are conspiracy theorists, many believe in ghosts and other supernatural entities, many are skeptical of science, etc. The ONLY common denominator that unites all atheists is that we do not believe in deities.
Theism is the belief in God or gods. What constitutes "God" or a "god" can vary widely depending on the tradition or even individual, but it usually(but not always) includes belief in the existence of a supernatural being. Theism is not the same as religion, which may be described as a set of beliefs, rituals, and practices regarding what is deemed sacred. Deism is a subset of theism; deists believe in a deity that does not interact with the world in any way beyond its initial creation. Pantheism is the belief that the world itself and everything it encompasses is God: you, me, trees, rocks, the galaxy hundreds of billions of miles away from us are all part of God. Pan-EN-theists believe this God extends outside of this universe.
Agnosticism is the position that absolute knowledge or certainty is unobtainable. This is not mutually exclusive with either Atheism or Theism; the majority of both are also agnostic. Agnosticism can also sometimes refer to a position between Atheism and Theism, a position that is neither a positive belief in the existence of gods, nor an inclination to their inexistence.
Gnosticism, with regards to Agnosticism, is the opposite of Agnosticism. Gnostics(in this sense) believe that absolute knowledge or certainty is attainable.
Other positions include:
Apatheism: As the name suggests, it is a position of apathy towards the question of the existence of deities. An apatheist does not care whether a god does or does not exist, they are de facto atheists, living life without regard to deities. The existence of God or gods is unimportant to the apatheist.
Igtheism/Ignosticism/Theological Noncognitivism: The position that the question of whether a "god" or "God" exists or not is a non-starter because the term is either too ill-defined, self-contradictory, or nonsensical to even begin to ask whether "it" might exist. The YouTube Channel TMM is a great example of this theological position.
PERSONALLY, I am an agnostic apatheistic atheist and maybe an igtheist. To put it into words: I am unconvinced that any gods exist(atheism), though I don't think it can be definitively proved either way(agnosticism), and as I see it, even if they did exist, they clearly have extremely little to no impact on my life and so the question of their existence is largely irrelevant(apatheism).