Arc length
A good way to think about arcs is the definition of a radian. If you take the radius of a circle and make it an arc length on the side of the circle, the resulting angle is one radian.
An arc length is always given by s = rš where s is the length and r is the radius. š must be in radians for this equation.
Circular sector
The circular sector is the area inside an angle defining an arc length.
It can be calculated with A = 1/2r^2š
Trig functions of real numbers
When you see sint or sins it indicates a trig function being evaluated on a real number, not an angle
These can be called circular functions.
The real number is treated as if it is a radian
For example: Angular velocity w = š/t
w = pi/2 / 10 = pi/20 rad/s