From NASA:

Wheels and a Destination

This view of the three left wheels of NASA's Mars rover Curiosity combines two images that were taken by the rover's Mars Hand Lens Imager (MAHLI) during the 34th Martian day, or sol, of Curiosity's work on Mars (Sept. 9, 2012). In the distance is the lower slope of Mount Sharp.

The camera is located in the turret of tools at the end of Curiosity's robotic arm. The Sol 34 imaging by MAHLI was part of a week-long set of activities for characterizing the movement of the arm in Mars conditions.

The main purpose of Curiosity's MAHLI camera is to acquire close-up, high-resolution views of rocks and soil at the rover's Gale Crater field site. The camera is capable of focusing on any target at distances of about 0.8 inch (2.1 centimeters) to infinity, providing versatility for other uses, such as views of the rover itself from different angles.

Image Credit: NASA/JPL-Caltech/Malin Space Science Systems

Interestingly, those holes on the wheels are Morse Code for "JPL".

.--- .--. .-..

Apparently using imprints in the dirt and the calculated distance traveled, they can determine how much wheel slippage is occurring.