It's a map site that helps you identify places around the world. Google maps is so commerce-focussed, Open Street Map often lacks an explanation of what something is.
But it clearly has issues such as not licensing the background options so it has watermarks and popups.
Opentripmap.com is similar, but is probably just OSM data.
Wikimapia is great for "what's that wierd marking in the desert"-type questions.