Notation:
Open interval (the end points are excluded)
(1,5) or 1 < x < 5
Closed interval (the end points are included)
[1,5] or 1 <= x <= 5
Read "<=" as "less than or equal to".
Or a mixture
[0,+infinity) or 0 <= x < +infinity
The above used "x" so represented domains; the range can be represented similarly by, for example, [-1,1] or -1 <= y <= 1.