After spending about 9 months in orbit around the Sun, the spacecraft will finally arrive at Mars. When it does, it is going too fast for it to orbit around Mars, so it has to slow itself down. This can be done by using thrusters, aerobraking, or a combination of the two.
When thrusters are used, the rocket is oriented in the direction opposite to its motion. Then, the rockets are fired so that the orbital speed of the spacecraft is reduced and it becomes slow enough to enter a Martian orbit. However, when aerobraking is used, the spacecraft uses its shape and aerodynamic drag with the Martian atmosphere to slow itself down. An initial rocket engine burn might be used to get it into an elliptical orbit and the atmosphere can provide the rest of the force to slow the spacecraft down.
Any way the spacecraft arrives Mars orbit, the next step of the Journey to Mars is the final one, Landing.
Journey To Mars 