How does a spacecraft get to where its going?
Retroburning and aerobraking are ways of slowing spacecraft.
Retro means backwards or behind. Retroburning means firing rockets in the opposite direction from the way they were fired to push the ship forward. This thrust in the opposite direction makes resistance to the ship's movement, and slows the ship down.
Aerobraking is a way of slowing down a spacecraft by using the atmosphere or outer gas layers of a planet. The ship "dips" into the atmosphere. As it does that, the molecules of gas in the atmosphere rub against the ship. This acts as resistance and slows the ship. Often a ship will use aerobraking to change an orbit or slow a ship enough so that gravity will pull it down to a planet.
This is a picture of the Mars Global Surveyor in the configuration it will be in when it aerobrakes. The thin, invisible top of Mars' atmosphere will rub against the ship, slowing it.
Photo from JPL's Planetary Missions & Instruments image gallery http://www-b.jpl.nasa.gov/pictures/browse/pmi.html
How can a spaceship leave orbit?
What is an orbit?
What causes an orbit to happen?
How does gravity work in space?
What are some different kinds of orbits?
How can we tell a spacecraft's speed?
How does speed affect an orbit?
What is resistance?
What is thrust?
How do spacecraft use an orbit to move from planet to planet?
What could cause an orbit to fail?