The Earth zips around the Sun at about 67,000 miles per hour, making a full revolution in about 365 days – one year on Earth. Mars is a little slower, and farther from the sun, so a full circuit takes 687 Earth days – or one Mars year.
That longer year means longer seasons too. Over-extended Martian winters, the shorter days and reduced sunlight mean that solar-powered spacecraft sometimes have to carefully conserve their energy.
Timing of Mars years is also important. Every 26 months, we come closer to Mars, so it's one of the best times to send spacecraft. A shorter trip means less time and fuel spent getting there.
When the Curiosity rover got to Mars, its mission was to explore Mars for at least 687 Earth days - one Mars year. But like our other long-lasting rovers, it hit its target and kept on rolling!