Adding to what Rei said, you have to differentiate people with AIDS and people who are HIV+.
Having AIDS (Acquired immune deficiency syndrome) means that the HIV virus is attacking and destroying your immune system. This means that your are susceptible to many diseases that would otherwise be harmless.
Being HIV+ only means you carry the virus. Sometimes, for many years, the virus will just be in your system. While HIV+ you may not have AIDS yet but you can still infect others with the virus.
The thing is that there are a couple of substrains of HIV which lead to AIDS. Most people with AIDS or that test HIV+ actually have a mixture of all of the sub trains. However, some people have an immune system capable of resisting to a specific substrain for an extended period of time. When you have 2 people that are HIV+, imagine that one (a) is resistant with the substrain that infected her and the other (b) has another substrain. The first person (a) would live perhaps 5 to 8 years without developing AIDS. If they have unprotected ***, the second person (b) will infected the first one (a) with the other substrain and will actually worsen the person's condition.
The main idea, as Rei said and very well, is to always have protected *** unless you are on a monogamous long relation and even so, its best to be cautious and use protection. (long relationship isn't a 4 months old teenage crush...)
I'd like to add that birth control pills, IUD, diafram and other anti conceptional methods don't protect you from diseases. The only safe way if to use a condom. Be sure to always carry a condom with you but be careful when you carry one in your wallet or your car. Heat (like the one by carrying your wallet on your back pocket or having your car in the sun) damages them and weakens them. There are some cases specifically designed to protect them a bit more than the normal package.
Also, a small notice to women: you should also always have a condom with you. Its something that should never be left only up to the guy, even if he is the one going to use it.
And finally, never ever ever trust 100% in a person you met a few hours (or less) ago, even if he/she is from your group of friends. Always use protection.
