I'm developing some software using ArcGIS API, but the question is rather theoretical, as I just came from Google Maps and have no idea where to start; everything seems to be different including the metrics used. I'm not even sure how to explain what I want to understand, so I'll just shoot (don't shoot me if I misname something).

So, the very basic understanding I'm missing is the coordinate system. I'm creating a MapView, then I assign an Envelope (this probably to be the area displayed on the map, right?) as its extent. The problem starts here, Envelope requires some parameters that are definitely not latitude/longitude (I've tried those and the map was showing some random area). From the samples I could just figure out that numbers used are rather strange (something like 658346.0152817797, 6239589.28803324, 841568.832957386, 6366554.416159659) and are not coordinates. What are these numbers? And how can I transform my lat/lon coordinates into acceptable numbers?