There are many, many, MANY different types of acid out there. The general and most popular description of an acid is:
Any compound that will dissociate into it's Anionic counterpart and a free Proton (H+ atom, that is a Hydrogen atom without its electron, which is basically a lone Proton) when subjected to water
You can have Strong acids like Hydrocholoric acid (which is what Otis posted and is one of the more popular ones), as well you can have Weak acids.
- Strong acids dissociate Completely in water. If you add a certain quantity of HCl in water, all that HCl will dissociate into H+ and Cl-
- Weak acids only dissociate partially in water.
(These are the Bronsted-Lowry definition of acids by the way, you can have Lewis acids as well which are pretty popular but just dumbed down to even more basic principles)What makes an acid strong or weak depends on how "mean" the atom (or group of atoms) attached to the dissociating Hydrogen is. If the atom or group of atoms tends to be allot more Electronegative then the poor Hydrogen atom, then they will pull all the electrons in the Atom/group -- H bond towards them, pretty much making the H a loser in this affair. So given the opportunity, H will go "forget you guys" and detach itself, leaving its electron behind, to live in a more stable form (Water helps stabilize H+, which is why acids dissociate in water).
Cl for example, is short for Chlorine. In pure form, it's a loving nasty gaseous compound that can easily melt your face and kill you. In short, Cl very strongly Electronegative and wants to keep any Electron it gets all to itself. Attach a much less electronegative Hydrogen to a nasty Chlorine and you get a fairly strong Acid that's ready to rip itself apart any time you give it some Water.
Now the nastiest acid I've ever seen is Triflic acid. It's fairly strong, so much so that it absorbs water from the atmosphere just to dissociate itself. Here's the wiki link to it
http://en.wikipedia.org/wiki/Trifluoromethanesulfonic_acidYou can also get Magic Acid, which is so insane you can't store it and need to make it on site by mixing specific reagents. I forget which ones though.
You can also have the polar opposite of acids, which are Bases (like soap, for instance, which is a weak base). These can actually get even nastier, as the strongest ones can eat through organic compounds (like skin, for instance). Only difference is that many strong acids can produce Halide Gasses (like Chlorine and Fluo0rine) which in themselves are very dangerous.
Edit: Bases dissociate into a Cationic counterpart and OH-
Mix Acid with Base and you get:
A-H + B-OH --> AB + H2O
Note, if AH and BOH are strong, you will never find A-H or B-OH in water, only their dissociated parts.
