Previously: Astronomers have known for a while (since 2002-ish) that there was water on Mars – but only in the form of ice, located on the polar caps of the planet.
Now: Why is liquid water such a big deal? It was previously thought that due to several factors (like the freezing temperatures and the thin, practically nonexistent atmosphere), that there would be no way for liquid water to exist on Mars. This discovery changes that!
What: The evidence for water is indirect evidence, which is to say that no one has yet to see the water itself. What has been found is a sort of stain that flows down the cliffs and craters. These stains are composed of hydrated salts, which can’t really exist without water. Visual and infrared data show that in the warmer periods these stains appear, grow, and flow downwards; in the colder periods they disappear again.
How: The environmental factors on Mars did not suddenly change, so how is there now liquid water when it previously was thought to be impossible? There are currently a couple of different theories floating around. One is that the ground is porous, and the water remains frozen in ground during the winter and in the warmer summer months melts out of the ground. Another strong possibility is that the salts found on the surface of Mars absorb water until they contain enough to run downhill. This explains the hydrated salt stains nicely. It’s a process that can be seen in deserts on Earth and the process itself is known as deliquescence. The salts themselves are actually the reason why it may be possible for liquid water to exist at all. Water on its own would not be able to exist in its liquid form on Mars, however a high salt content prevents the water from freezing except at extremely low temperatures – this is why roads get salted in the winter.
Future: Apparently the biggest problem of getting to the water will be trying not to contaminate it with bacteria from Earth, as earth contains a wide variety of bacteria that doesn’t require oxygen, or warm temperatures, and could happily flourish with a bit of damp soil.
Source: GIF from Nasa, info from The Guardian, The Atlantic, IFL Science, and Gawker.