Circular templates create a radius around the starting point.
Cones create an effect in the shape of a triangle or pizza slice from the starting point.
A rectangle uses the origin point as one of the corners, treating the origin as being inside of the rectangle's area.
A ray creates a single line that is one square in width and as long as you want it to be.
An enumeration of the allowed types for a MeasuredTemplate embedded document
See
https://foundryvtt.com/article/measurement/