Providing more personal information than is absolutely necessary. Typically done when two or more people are conversing and details of one's sexual life creep into the discussion - or overly gross and disgusting details are included. Sometimes used in reference to loud cell phone users.
"dude, you know how like when I eat eggs and it gives me this horrible gas? Well, like I really had to go and I couldn't sit down on the toilet seat because the last guy left some crap on the seat and so I didn't want to touch it so I hovered and it came out and hit the wall and..."
Friend: "Dude, that's oversharing."