Many people ask me what it is to be Straight Edge. well im not very good with words so this a description that i found in a forum once. sorry i dont know who wrote it.

Straight-edge refers to a philosophy that's most basic tenets promote a drug free lifestyle. It developed as an offshoot of the punk rock/hardcore scenes of the early 1980's when the term itself is believed to have been coined by Ian Mackaye, in the self titled song, while he was the singer of the seminal hardcore band, Minor Threat. Mackaye eschewed the nihilistic tendencies of punk rock, promoting instead the simple (almost simplistic) philosophy of "don't drink/ don't smoke/ don't fuck."
In the 12 years since the demise of Minor Threat, these simple beliefs have transformed the minds of scores of teens worldwide. Increasingly disenchanted with societal ills, young men and women adopt the straight-edge doctrine as a blueprint to better first themselves, and then the world in which they live. While the original definition of straight-edge only included the rejection of mind altering substances and promiscuous sex, modern interpretations include a vegetarian (or vegan) diet and an increasing involvement and awareness of environmental and political issues.

As noted, straight-edge grew out of the punk rock/hardcore scene and so music plays an important role. The Teen Idles, an early 80's Washington, DC hardcore band, and something of a precursor to Minor Threat, can arguably be called the first straight-edge band. Since that time there have been hundreds if not thousands of bands who've so labeled themselves. Early bands included Minor Threat, SSD and Uniform Choice. In the mid to late 80's, straight-edge hardcore reached a zenith, especially in the greater Metropolitan area of New York City. This atmosphere led to the creation of Gorilla Biscuits, Bold, Wide Awake and arguably the most prolific band of the era, Youth of Today. While the bands of this period did much to popularize straight-edge,they also contributed to its closeminded and antagonistic aspects. The attitude between straight-edge and the rest of the world often took on adversarial tones during this time, largely becoming "The positive youth crew versus people who drink, smoke and/or do drugs." Many people dislike straight-edge and its adherents because of such intolerant views.

Straight-edge today, while nothing like the "halcyon" days "back in '88," still offers a viable and positive lifestyle. Bands such as Strife, Mouthpiece, Earth Crisis and Snapcase, continue to proliferate and bear the standard. While its detractors often claim that straight-edge(rs) are no more than suburbanite, cliquish, fashion victims, few can argue that the philosophy is still valid. The drug-free lifestyle has left its positive impact on more than one wayward youth. Perhaps the ideals and ideas are more pertinent today than ever as the focus begins to reach beyond affecting merely oneself to altering and improving one's society and environment.
between straight-edge and the rest of the world often took on adversarial tones during this time, largely becoming "The positive youth crew versus people who drink, smoke and/or do drugs." Many people dislike straight-edge and its adherents because of such intolerant views.

and to add on from another source

Straight edge is a life long oath to abstain from all forms of recreational drugs including alcohol and cigarettes, to abstain from promiscuous sex and moderate our sexual desires in general. These are the only guidelines for straight edge; all other life choices such as veganism, Christianity, Buddhism, Krishna , hardline, vegetarianism and animal rights among others are all optional pursuits often associated with straight edge. Straight edge is an independent philosophy and is not directly associated with any creed, trend, music, religion or principle. Straight edge was born out of the hardcore music scene but the ONLY requirements of adopting a straight edge life is abstention from drugs and promiscuous sex. I AM STRAIGHTEDGE The straight edge oath is one for life, taken to the grave (“til' death”. Straight edge is not for everyone. Straight edge is not flexible. Straight edge is a radical commitment to stay free of addiction and the destructive escapism common to society for your entire life. If you cannot maintain this oath until death, simply do not take it. If you must have promiscuous sex, call yourself “oison free.” The abstention from promiscuous sex is an element of straight edge often ignored or casually looked over by straight edge people but it is equally as important as not drinking, smoking or drugging yourself.