Those who make everything about something sexual, those who look at a girl's breast when talking to a girl, and those who like looking under a girl's skirt are examples of perverts.
Someone who's idea of normal sexual interaction just happens to be kinkier than yours.
Example: (While having sex outside on a park bench)
Look over there honey. Those people having sex on the marry-go-round. I can’t believe them. They are such perverts.
Someone who's constantly got sex on the brain, and usually loves to share it with others.
John: HiJane.
Jane: Hi John. Hey, look. A dollar on the ground.
John: Oooh, really? *Bends to grab $*
Jane: Biggety back that ass up!
John: You shameless pervert.