Typically a high school kid who is greasy and smells bad. Other characteristics that constitute being a bucket are being under-educated (or act in a manner that suggests so), being a dirty looking whore and altogether unkempt looks. Another way of saying it is 'buck'.
Person 1: "ew look at that greasy bitch."
Person 2: "What a bucket."
Person 1: "Man, there's so many gross kids in here."
Person 2: "What a bunch of bucks."