A square root is the result of "unsquaring" a number, or the reversal of the process of squaring. The root is, as its name implies, a squared number reduced to its most basic form.
A square is just a number multiplied by itself. For example, "4 squared" is equal to 4 x 4, or 16. Square roots are the same function, only inverted: the square root of 16 is 4. In this sense, square roots are analogous to subtraction and division. Root numbers represented by whole numbers are considered to be perfect squares. Nonperfect square roots can only be rendered in irrational numbers, or numbers with an ongoing decimal remainder. Nonperfect squares are usually written as approximations.