Acids and bases are everywhere in chemistry. Acids taste sour and can dissolve metals. Bases taste bitter and feel slippery. The pH scale measures how acidic or basic something is, from 0 to 14.
pH 7 is neutral - pure water. Below 7 is acidic, above 7 is basic (alkaline). Your stomach acid is around pH 1-2. Bleach is around pH 12.
The Chemistry Behind It
Acids increase hydrogen ion (H+) concentration. Bases increase hydroxide ion (OH-) concentration. When they're equal, you get neutral water.