A grad — also known as a gradian (not to be confused for the gradient of a function), grade or gon — is a unit of angular measure. It is largely deprecated today, and is not used in any mainstream community.
Grads may be considered a more convenient unit of measure than the degree (especially for mental arithmetic) because the number of grads in a circle is 400, a "nicer" number than 360 (the number of degrees). In fact, any grad measurement divided by 4 yields the percentage of a full circle (or revolution). 100 grads is a right angle. The grad was invented to replace the degree, but failed to catch on.
The grad, like the degree, is an inconvenient unit in higher level mathematics and the sciences; instead, the more mathematically sound unit called the radian is used.