What atheists believe in is the same types of things that everyone else believes. Atheists don't believe in a higher power or in the supernatural in general. Faith is put into science, people, and hope for a better future. The difference is that an atheist requires some sort of verifiable evidence or methodology to support such faith. If these facts change, so do the beliefs of the atheist. This can be frustrating to a traditional believer who relies on the never changing dogma of their particular religion. Religious beliefs come more from some sort of holy text that probably has no way to provide any evidence for their statements or rules. With that said, there are two basic types of atheists, strong and weak.
The rarer of the two is the strong atheist. This type of atheist believes with certainty that there is no god. This believe is mostly derived from the lack of evidence for any higher being or force. Because there has been no reliable evidence for so long and the fact science has debunked most or all of the preconceived ideas, strong atheists feel confident in their belief that no gods exist. Strong atheists have the unfortunate habit of being the most militant about their position and can be abrasive towards believers. There aren't many strong atheist as this type of certainty can be uncomfortable for most. How certain can you be of anything, let alone that in the entire universe there is no god? Even weak atheists argue this fact with the strong side of atheism.
The weak atheist, or sometimes called the agnostic atheist, believes there is no evidence for god so they can't justify believing in a god. This may confuse people. The weak atheist doesn't believe there is a god because there is currently no valid evidence to support that conclusion. They would change their minds if evidence were presented that satisfied their standards. Most atheists fall into this category of atheists. People who call themselves agnostic also fall into this set, because agnostic just means a lack of knowledge. If you have no knowledge of a god, then by default, you don't believe in a god.
What atheists believe in is largely dependent on what can be proved at the time. Atheists are skeptics and don't follow the crowd. You could get an atheist that believed in alien abductions or Big Foot but they are pretty rare. Also, facts change in science and history and so will a skeptical thinker change their mind. An atheist is much more fluid in their stances and beliefs and vary from person to person. This is why you see such a variety of people that call themselves atheist. They are much harder to pin down than say your average believer. If someone says they are Christian, then most people have a general idea about them and the culture they came from. It is the idea of what can be supported as true is what defines the belief system of an atheist.