A tile is simply a square graphic file (PNGs in our case) that, together, make up a tile set.
When you look at a Google Map, you’re actually looking at a bunch of tiles (think The Brady Bunch opening where each Brady head is a tile). Google Maps figures out the boundaries of your map against a vast set of images and serves up only the tiles needed to draw the map you’re looking at.
These tiles are stored and pulled from a tile server. Google has theirs, and you can easily create one for yourself to draw whatever you want over the map.