Another point "x" and "y" are typically used in a rectangular coordinate system and "r" and " " are used in polar coordinates. You can, of course, use whatever letters you wish.
The graph of "y= sin(x)", in rectangular coordinates is the "sine wave".
The graph of " ", in polar coordinates is the circle with center at (0, 1/2) and radius 1/2, as goes from 0 to .
But, again, the difference is because of the coordinate systems, not what the variables are called.