(for those under 30 years old) An intransitive verb indicating a negative state of affairs. Could be poor performance, bad weather, illness, general dislike, or any of innumerable negative thoughts.
(for those over 30 years old) Always a transitive verb. Colloquially used as shorthand for suck dick with noun dick understood. Synonymous with blow and bite. Used as an insult; indicates homosexuality.
(under 30) Cracka 1: Hey did you see the Simpson's last night? Bart said his father sucks!!!
Cracka 2: So, what's your point?
(over 30) Son: Gee dad, I don't want to do my homework. Math sucks.
Father: You're grounded for a week, you perverted little delinquent. And wash your mouth out with soap.