Not to be mistaken for an atheist who merely uses the internet, an internet atheist is someone who is ubiquitous when it comes to websites or forum threads related to religion. They will often poke fun at religion and religious people, especially Christians and Muslims, but they have a sort of "search and refute" thing going on with Christians, whereas the Muslims usually only fall prey to internet atheists when they happen to come across them. Of course many internet atheists have the habit of searching for Muslims, too.
They usually lack any sense of humor if jokes are made about them, but they find nothing wrong with being incredibly offensive to the point of being disgusting when making jokes about religious people. They also love to show off their knowledge about any subject they might know something about.
They will often make snobby attempts to annoy or insult religious people, sometimes calling them names, but will immediately condemn someone who bothers atheists, homosexuals or liberals if they don't have a legitimate reason. Even if they do have a legitimate reason that justifies their bothering it might not matter. They will also claim they don't discriminate, but they are usually far more polite and accepting if they think you're a fellow atheist.
Jane the Theist - "God sure has given us a fine day. He did a great job creating the birds."
Internet Atheist - "'God' didn't give as anything; 'God' doesn't exist. And the birds are a product of years of evolution and natural selection."
Red the Atheist - "Christians are morons. They sicken me."
Joanne the Christian - "Ya? Well atheists are jerks."
Red the Atheist - "You're making nonsensical generalizations!"