Reflection over the y-axis
A reflection is a type of Rigid Motion. On a coordinate plane, a reflection is depicted by the movement of coordinates over a line of reflection. Without the coordinate grid present we can tell that a reflection is precise because each image and it's corresponding pre-image is equidistant from the line of reflection and is across the line at a 90 degree angle, and therefore perpendicular to the line.

Rotation on a Coordinate Grid

The image is rotated about the origin 90 degrees. We note that it is a clockwise rotation, otherwise it is assumed to be counterclockwise due to the positive angle measure.
Rotation around the origin