A datum is a theoretical construct that first relies on creating a potato-shaped object called a "geoid", then from that, constructing an ellipsoid which will serve as the basis for the datums to operate on. It is a way of projecting the globe and preserving area and distance (shape as well, depending upon scale and format), so that we may use GPS and other technologies.
There are hundreds of datums used worldwide, but WGS84 was the latest one to gain widespread use. It was conducted in 1984 by the U.S. Department of Defense and now used in almost every country in the world. It uses the Earth's center of mass (determined from satellite technology) as a benchmark.
That was probably more than you asked for, but that's a brief overview of what a datum is.
It looks like I misspoke earlier, as I don't see a way to switch datums on the fly with Google Maps, but it looks like Navfree (free on Google Play) supports a bunch of different countries using an Open Street Maps sort of format.
Keep in mind that data from places like Open Street Maps does not face the same level of verification and scrutiny than that produced by cartographers, so always be cautious of your surroundings even when using a GPS.