ACIDS AND BASES ARE EVERYWHERE
Look around you and every liquid you see will probably be either an acid or a base. The only exception would be distilled water. Distilled water is just water, that's it. Most water you drink has ions in it, and it is those ions which make something acidic or basic.
In your body there are small compounds called Amino Acids, those are acids (Duhh). In fruits there is something called Citric Acid, that's an acid too (Duhh again). But what about baking soda? When you put that in water it makes a base. Vinegar? Acid.
Scientists use something called the "pH" scale to measure how acidic or basic a liquid is. The scale goes from "0" to "14". Distilled water is 7, right in the middle. Acids are found between "0" and "7". Bases are from "7" to "14". Most of the liquids you find every day have a pH near "7", either a little below, or a little above. When you start looking at the pH of chemicals, the numbers go to the extremes. If you ever go into a chemistry lab, you could find solutions with a ph of "1" and others with a pH of "14". Those chemicals are very dangerous. There are pH values higher than 14 and lower than 0, but let's just start with 0-14.
NAMES TO KNOW
Here are a couple of definitions you should know...
ACID: A solution that has an excess of H+ ions. It comes from the latin word "acidus" which means "sharp".
BASE: A solution that has an excess of OH- ions. Another word for base is ALKALI.
AQUEOUS: A solution which is mainly water. Think about the word aquarium. AQUA means water.
STRONG ACID: An acid which has a very low pH (0-4).
STRONG BASE: A base which has a very high pH (10-14).
WEAK ACID: An acid that only partially ionizes in an aqueous solution. That means not every molecule breaks apart. They usually have a pH close to 7 (3-6).
WEAK BASE: A base that only partially ionizes in an aqueous solution. That means not every molecule breaks apart. They usually have a pH close to 7 (8-10).
NEUTRAL: A solution which has a pH of 7. It is neither acidic nor basic.
WHAT REALLY HAPPENS
A little tricky here. The straight answer...
Acids are compounds which break into Hydrogen (H+) ions and another compound when placed in an aqueous solution. Bases are compounds which break up into Hydroxide (OH-) ions and another compound when placed in an aqueous solution.
So, now the CHEM4KIDS way. If you have an IONIC compound and you put it in water it will break apart into two ions. If one of those ions is H+, then the solution is acidic. If one of the ions is OH- the solution is basic. There are other ions which make acidic and basic solutions, but we don't talk about them here.
That pH scale we talked about before is actually a measure of the number of H+ ions in a solution. If there are a lot of H+ ions, the pH is very low. If there are a lot of OH- ions, that means the number of H+ ions is very low, so the pH is high.
That's basically it. (Ha Ha, get it?)