To summarize:

We've known for very long that the Mars polar caps contained water ice (since Mariner 9 and the Viking orbiters in the 1970s). We've known that the ice sublimates and deposits on the surface during local winter at high latitudes (Viking landers, 1970s). We've known that Mars had substantial amounts of liquid water on its surface at least for transitory periods (Mariner 9 & Viking orbiters, 1970s).

Since the early 2000s we've known that Mars contains substantial amounts of water ice in the form of sub-surface permafrost. Specifically that much of the surface of the planet, if not all of it, is underlain with permafrost that is as much as half water ice in some areas, at a minimum depth below the surface of only around a meter. This permafrost layer exists even in middle latitudes and might exist even at equatorial latitudes at a greater depth.

During the same time frame the Spirit and Opportunity rovers provided evidence that Mars experienced extended periods (of at least many millions of years) where substantial quantities of liquid water existed on the planet, leading to the formation of various geological features and minerals which could only be produced under those conditions.

More recently (within the last 10 years) the Mars Reconnaissance Orbiter has provided evidence of the presence of sub-surface glaciers on Mars. These glaciers are vast volumes of water ice under thin layers of loose surface soil (a few meters to just one meter), and they exist all over Mars at latitudes as low as 30 deg. To put that in perspective, that's about the same latitude as New Orleans and San Diego.

Even more recently (this year) data from the Mars Express orbiter pointed to the existence of a lake of liquid water 1.5 km beneath the Southern polar cap, likely made up of very briny water.

All of this points to several interesting conclusions.

Firstly, in regards to future colonization of the planet by humans there appears to be abundant and easily accessible reserves of water ice throughout much of the planet. This means that there are a great many (instead of very few) prime locations on Mars for placing settlements. With water ice and the CO2 atmosphere you can make: water (of course), oxygen, hydrogen, and methane which makes it possible to manufacture key resources for interplanetary transit (LOX/methane as a rocket propellant), human habitation (water and oxygen), farming (water, CO2, oxygen, sunlight), energy storage and fuel for vehicles (oxygen and hydrogen or methane), and even the beginnings of industry (plastics from methane, iron and steel from carbon monoxide et al, glass, aluminum, etc.) Being able to site settlements at mid latitudes means being able to choose areas that have decent sunlight most of the year, making it possible to grow crops using natural sunlight and to use solar power as at least one component of power generation.

Secondly, in regards to the history of Mars the evidence of the presence of liquid water over geologically significant time periods combined with the evidence of pockets of sub-surface liquid water existing even into the present day means that Mars may have harbored environments suitable for the formation and existence of life, and some of that life may actually still exist through to the present day.