IRL, I'm fairly reserved and introverted. I hate crowds (I need alcohol to get through family gatherings, but I doubt that's unique to me), and talking for too long actually emotionally drains me. It's difficult for me to speak for multiple reasons. I like laughing and joking around, but I only do that with people I trust.

Online? Everything I repress comes out. I enjoy trolling (though I am very aware of lines that shouldn't be crossed), especially my friends. 

Suffice to say that if any of my internet friends met me IRL they'd be surprised at the difference. I'd say the same about the vice versa, but that would require IRL friends.