What is the difference between an acid and a base?
Base is anything, which has a capability to abstract a proton.
Using the easiest definition, an acid is something when added to water reduces hydrogen ions (H+), also called protons. A base, or an alkali, is something that when added to water reduces hydroxide (OH-) ions.
The strength of a basic (or alkaline) or acidic solution is calculated using the pH scale. A pH of 7 is perfectly pure neutral water (neither acidic nor basic), and pH below 7 is acidic, and a pH above 7 is basic.