Earlier this month, President Obama proposed a new budget for NASA that would see Mars funding cut to just $189 million in 2015. This isn't enough for any big missions, but NASA has decided to shuffle some things around and see if they can't pull enough money together to make something happen by 2018.
The reason for the 2018 window is simple: to send something to Mars from Earth most efficiently involves using what's called a transfer orbit, where a spacecraft leaving Earth uses just a little bit of thrust to diverge its orbit away from Earth's and out towards the orbit of Mars. For this to work, the spacecraft has to aim for a point out in space where Mars will be by the time the spacecraft gets there (as opposed to where the planet is currently), which means that both Earth and Mars have to be set up just right relative to each other. This happens approximately once every two years, with a super-efficient launch window every 16, and 2018 will be one of these extra-good ones. And the more efficient the window, the more stuff we can send and the faster it'll get there.
NASA's new idea (as discussed by NASA science chief John Grunsfeld last week) is to take advantage of the 2018 window by scraping together funds from the human spaceflight program and the space technology division to come up with about $700 million to make something happen in the next six years. $700 million isn't a huge amount of money once you blow a bunch of it on a launch system, but it's definitely enough for a decent orbiter, and there's a chance that enough could be found by 2020 to send another rover.
Speaking of rovers, we should definitely not forget that between now and 2018 (this August, in fact) Curiosity will land in Gale Crater, and if it does as well as Opportunity has, it may very well still be around by 2018.