PSA: O Negative is universal donor; AB Positive is universal recipient
O Positive can be given to men or to women past child-bearing age, but usually is done only in a dire emergency and with physician or Pathologist approval.
I saw the blood type thread at work this afternoon (slow day); I felt this was important enough to give it a thread so it wouldn't get lost in the shuffle. Ymmv.
I'm a medical lab technician, so do routine blood typing, antibody screens and cross-matches as part of my job.
For anybody who cares, here is a summary of what the typing stands for.
Red blood cells have a variety of proteins in their membranes called antigens. Because they are part of our body's system, we normally don't develop antibodies to the antigens we have and only develop antibodies when exposed to foreign antigen, usually but not always through transfusion. (A reminder: antibodies are part of the immune system, and function to flag and rid the body of "not self/foreign" invaders. Blood antibodies float around in the plasma.)
ABO System
However, A and B are antigens that are apparently similar to to something in nature, possibly a bacteria, that cause us to develop antibodies to A or B shortly after we're born if we don't have them on our cell membrane, that is if they are "not self."
People who are Type A have only A antigen and shortly after birth will develop anti-B antibodies through environmental exposure. If you give them Type B red cells, their anti-B antibodies will bond with the transfused Type B blood cells and cause a severe, likely fatal, transfusion reaction.
People who are Type B have only B antigen and shortly after birth develop anti-A antibodies through environmental exposure. If you give them Type A red cells, their anti-A antibodies will bond with the transfused Type A red cells, and cause a severe, likely fatal, transfusion reaction.
People who are Type O lack both A and B antigen, and shortly after birth develop both anti-A and Anti-B antibodies. If they are transfused with A, B or AB, they will have a severe, likely fatal, transfusion reaction. However, because they lack A and B antigen, then can be transfused into A, B or AB because they don't have anything for the patient's anti-A and anti-B antibodies to bond with.
People who are Type AB have both A and B antigen, and do not develop A or B antibodies. Therefore, you can give them A, B or AB and they won't have a reaction. However, you can't give them to anybody else because they bond with their anti-A and anti-B antibodies.
Positive versus Negative
People who are (Rh) Positive have D antigen and don't develop D-antibodies. People who are (Rh) Negative (for D) lack D antigen and may develop anti-D antibodies if exposed through transfusion or through pregnancy.
Antibody screen
There are other less common antigen families on our red cells to which we can develop antibodies if we're transfused or sometimes through certain illnesses. The antibody screen tests for the most common ones. Patients that test positive to the screen are further tested to identify the antigen(s) and if they need to be transfused, we order special blood proven to be free of that antigen from the American Red Cross.
Oh, and going back to the other thread, I am an extremely rare subtype of A Negative, so my blood isn't real useful for donation, and as a recipient I would require O Neg. That's a whole 'nother level of detail on the above. 
= new reply since forum marked as read
It's full of excellent facts that we all should be exposed to.
I'm bookmarking it in case I run across any other threads where this info would be useful.
I was told long ago that I had developed a very rare antigen from multiple transfusions, and that I could be transfused only with blood that is negative for that antigen. They even gave me a card identifying the antigen, which was a long string of letters and symbols. I was told only that the antigen is extremely rare, and that it was being studied by the W.H.O. at the time (late '70s).
It's nice to finally have a clue about what it all means, lol. Thanks for the clear explanation. 
even with a carrying card.
Every patient is treated as a new patient when it comes to type, screen, antibody identification and crossmatch. We don't even trust co-worker's testing because the consequences of an error can be fatal.
(btw, just to clarify, you developed an antibody to the foreign antigen. 
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I lost the card years ago.
And yes, I meant I received the antigen and developed the antibody.
Thanks, again.
But you can get blood from anybody.
you can NEVER be given the wrong blood type - you can have 'em all!!! The O neggers, who can GIVE to everyone, can only receive O neg except for certain circumstances
you are right that your blood is not coveted, but your platelets certainly are - if you ever want to give, try donating platelets 
I'm a 45 gallon platelet donor
I really don't do anything but lay there and watch a movie; easiest way I can think to "give back" to the community (that and donate to the local food bank) I don't find what I do amazing in any way; what I find so amazing is how few people donate to the blood bank yet always expect the product to be available for them and their loved ones 
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It's good karma!
It's a form of O positive, but the D antigen is so weak that it barely registers. As a donor, I'm considered O positive, but as a recipient, I'm considered O negative. Different blood typing agencies have had different policies about that over the years. At one time I had a blood type card in New York state classifying me as O negative while I still had one from California classifying me as O positive, and these weren't oversights -- these were correct according to the standards (at that time) of the two different agencies. 
We need a smilie for that! 