Here are some G Codes:

G0 X Y:Go linear to X,Y position with max speed

G1 X Y Z F: Go linear to X,Y,Z position with speed F

G2 X Y Z R F : Go circular counter wise to X,Y,Z position with speed F and radius R

G3 X Y Z R F : Go circular counter clock wise to X,Y,Z position with speed F and radius R

These are the most basics. Further more there are a lot of G codes (also known as ISO codes).