atheist means to reject god. agnostic means to be neutral about god. babies don't reject god, but can be said to be neutral.
Neither of those are particularly accurate. As others have stated, but I think it's important to understand why
the words mean what they do.
Firstly, all -ism words are about beliefs. They denote world views, philosophical systems or idealogies.
Secondly, a- means "not"; it is a negation.
So what both words have in common is that they are a type of belief involving a negation of some kind. What kind?
For atheism, the root here is "theos" meaning "god." Hence: "belief in no god." This can mean an active or passive disbelief, each flavor of which has earned its own labels over time (positive/strong, negative/weak, etc.).
For Agnosticism, the root here is "gnostos" meaning "to be known" leading us to "belief in no knowledge." But, the context in which the term was coined, it is belief in no knowledge specifically of god (and other elements associated with a god).
The terms are related in the sense that they both deal with people's beliefs of god, but they address different aspects of god. Atheism tells you that the person doesn't believe in a god. Agnosticism tells you that the person believes that one cannot attain knowledge about god. There is quite a big difference here. You can believe or disbelieve in a god independent of whether or not you think knowledge can be obtained about god. Here are different ways that can manifest:
Agnostic atheist - No one can ever know whether or not a god actually exists, and I don't believe in one.
Agnostic theist - No one can ever know whether or not a god actually exists, but I believe in one anyway.
"Gnostic" atheist - You can know whether or not a god actually exists, and I don't believe in one.
"Gnostic" theist - You can know whether or not a god actually exists, and I do believe in one.