I just covered this last semester (I am now entering my junior year of nursing school) -- let's see how much I remember...
There are two kinds of acidosis and alkalosis: metabolic and respiratory. In respiratory acidosis, I believe that the CO2 levels are high, while in respiratory alkalosis, the CO2 levels are low.
In metabolic acidosis, bicarb is low, while bicarb is high in metabolic alkalosis. CO2 levels remain the same.
Hopefully, someone more experienced will correct my mistakes if my advice is wrong.
It is important to know about acidosis and alkalosis b/c it can inform you if the lungs are working properly and hence if the rest of the body is (since the body is sluggish without proper O2 levels). It goes the same for metabolic acidosis and alkalosis -- too much or too little of a bicarb means something is out of whack in the body and that the person needs to be checked out since he obviously isn't metabolizing bicarb properly.
This is what I recall after months of not touching a nursing school book. Hopefully I have not led you too far astray!