Interval কী?

Let $a$ and $b$ be real numbers with $a < b$. An interval is the set of all real numbers between two bounds. A square bracket means the bound is included, a parenthesis means it is excluded:

The symbols $-\infty$ and $+\infty$ (“infinity”) are not numbers, only a way of saying that the interval continues forever; the bracket next to them is always a parenthesis. The whole line $\R$ is the interval $\intoo{-\infty}{+\infty}$.