One who has made a lifetime commitment to not smoke, drink, do drugs, or participate in promiscuous sex.
This does not mean no sex at all, it just means only sex when in a serious relationship - not random hook-ups.
Being straight edge has nothing to do with religion. Although someone who is religious can be straight edge - the two are not correlated.
One cannot call themselves straight edge unless they plan to obstain from these acts for life. If someone breaks one of these, he or she was never straight edge.
Those who are straight edge feel they do not need to smoke, drink, do drugs, or participate in casual sex to have a good time and enjoy life. People who are straight edge still go to parties, go see bands at shows, and most of what non-straight edge individuals do - they just don't poison their bodies in the process.
I'm 25 and have been straight edge for 8 years - I go to parties, still see my favorite bands, and have sex with my husband regularly. I never engage in acts that I regret since my mind is clear at all times. I don't cough up a lung going up the stairs since I don't smoke. Being straight edge has never stopped me from having a good time - and the best part is that I remember all of them!