Definitions In My Own Words
An Agnostic is someone who doesn’t feel there is enough evidence to prove or disprove the existence of a deity. Being an agnostic also includes the belief that whether or not a deity truly exists could be something that is unknowable, or not meant to be known.
An Atheist is someone who does not believe that there are, or ever have been, any deities responsible for our universe or anything in it. There are no other static, unifying traits among those that identify themselves simply as such.
A Humanist is someone who believe that humanity as a whole is more important than any one individual group. As Spock said, “The needs of the many outweigh the needs of the few.” In my mind you can be a humanist and not be an atheist, although I believe this to be the rarest of species. If someone has a belief in a deity (or deities) that doesn’t contradict the basic idea that no one group (saved vs. unsaved, christian vs. not christian, ect.) has more favor with said deity, or has any way to invoke more favor (i.e. prayer), that does line up with the basic beliefs of the humanist movement. In addition, humanists believe that we are responsible for our own actions and choices. There is no destiny. There is no divine intervention. Everything happens for the sole reason that a series of individual and collective human decision have made it a reality. So if someone believes in a deity that can and does intervene based on the pleas of human to do so, that does not not line up with the beliefs of the movement.
A Skeptic is someone who requires a information to be well supported by evidence. People who identify themselves as skeptics do not hold a bias for reality to be one way or another other; they simply want reality to prevail. A person who identifies them self as a skeptic can also be a humanist so long as the desire to continue to embrace factual evidence is the most prominent. Presently, there is no well supported evidence for a deity, thus all Skeptics are also Atheists.
A Theist is someone who believes that at least one deity exists. This term encompasses anyone who believes in a God or Gods as described and defined by modern day Christianity, Judaism, Islam, Sikhism and a variety of Pagan religions. It also includes people who believe in an abstract God or Gods and people who identify as “deists”.