State two types of functions that you know that do not have a maximum or a minimum (unless the domain is restricted to some interval)

Correct me if I am wrong, is it:

f(x) = √x (sqrt)

f(x) = x

f(x) = 2^x

I don't understand when it says "unless the domain is restricted to some interval."

Thank you~

Edit: Posted in wrong section, mods please move this thread to pre-calc section~