Let's start with the basics. Testosterone is "the male" sex hormone (quotations because women actually do produce small amounts of it, as do men produce small amounts of estrogen for balance). I'm no chemist, but I did get a B in high school chemistry. In case you actually care about it, 17-beta-hydroxy-4-androstene-3-one is testosterone. Testosterone is produced in the testes (or the balls, as I was so eloquently told by a friend explaining it to me in high school) and is involved in starting puberty.
Benefits range from the obvious to the unseen. Unless you've been living under a rock, you know injecting testosterone in yourself will increase muscle mass, which is just one of the benefits. Others include libido, aggression, longevity, increased metabolic rate and energy. It is also noted in studies that with low testosterone come higher rates of depression. Higher levels of testosterone have shown to lower anxiety levels (my interpretation is because you're too busy being a fucking man).
While testosterone is positively correlated to levels of aggression, that doesn't mean to be a shithead and try to fight everyone you see. Most of the bad rap on steroids stems from people who aren't intelligent enough to use it. They have a short temper, add more testosterone and freak out over the stupidest shit. Then they forget to regulate their hormones after their cycle is over and think they are a woman trapped in a man's body (yes that does happen on occasion, but only for the terribly uneducated).
Maybe its just me, but maybe testosterone shouldn't be as taboo as people make it out to be. We're too busy being politically correct and pandering to the weak instead of telling them to "man up".
I'm too lazy to cite my sources, but Google and Google Scholar helped me out.
Who needs sources with a pic like this?
I finally wrote this while I was bored at work, so let me know if it sucks/if you catch any errors.