Someone who talks or dresses in an unusual style and makes no attempt to conform. A loner maybe with little or no social skills who is awkward around people. Stalkers would be weird too as they think it's okay to behave like that when it's actually really creepy.
I am definitely a weirdo. I'm a loli loving bisexual anime otaku. And that is just the tip of the iceberg. I'm 19 and have an imaginary friend, I dropped out of highschool at 18 in the tenth grade and I fantasy role play on facebook. So you, basically anyone like me.
My definition of a "weirdo" is when someone breaks the barriers of social normality, and behaves in a manner different from the "socially accepted standards". So for example, not brushing your hair or teeth, is a bit weird, MOST PEOPLE consider that as a normal lifestyle.