People who show up to parties and don't pay for anything. They steal your beer and liquor without paying. You can spot a slider by hearing phrases such as:
"What kind of beer do you guys have?"
"Dude this party is awesome, can I have a beer?"
(walk in the door) "Next on the table"
(walk in the door) Amy: Hey guys sweet party
Amy: What kind of beer do you guys have? (holds a beer examining it)
Amy: Cool, next on the table!
Dave (to chuck): Sliders...