Omnisexuals are attracted equally to all human beings (men, women, transexuals, etc.) They tend to go through phases of gender attraction that switch frequently and will eventually settle down with whoever wins their heart, regardless of said person's gender.
Basically, it's a broad term used to refer to any and all sexual orientations, expressions, and so forth. From homosexuals, to transgenders, to sometimes people with foot fetishes, someone who is omnisexual will love you for who you are.
(Based off my own definition of the term)
One of the doctors in Doctor Who *Confirmed*
Myself (Is an omnisexual)
"Dude, that lamp post is so sexy."
"You know what I've always found really attractive? String Theory."