Relations are equations that produce graphs eg straight lines, parabolas, sin+cos+tan curves, exponential curves, hyperbolas, circles, ellipses, square root, lol, ln (natural log), cubics and quartics etc. Some graphs like a circle might go as far as -3 to 3 on the x axis. Then we say the domain is [-3,3] It means all values of the equation can only be between and including -3 to 3. For most of the above common graphs the x values go from -ve infinity to infinity. If you mad a graph of y=sqrt(x), you would find the domain is [0, infinity) because no -ve x values "work" ie there is no sqrt (-4) If we were studying what y values the equation could have, we would be studying the "range" . helpful ?