- Base yield: +1 Food and +1 Gold
- Provides Fresh Water to nearby tiles
- Can provide Fresh Water via an Aqueduct
- May contain sea-based Resources, if the lake is big enough
- Allows construction of Harbor District
A relatively small (up to 6 tiles) expanse of shallow water which is surrounded entirely by land is considered a lake. Their main function is to provide a source of fresh water for cities founded next to them (or connected to them via an Aqueduct); this is also their main differential from Coast. However, if they are big enough, they will almost always have at least one sea resource and can thus be explored just like seas and oceans.
The Huey Teocalli Wonder is specifically designed to enhance Lakes. It adds +1 Food and Production to all Lake tiles for a total yield of 2 Food, 1 Production and 1 Gold, plus any additional bonuses from Resources (and bonus Amenities for the city which built it)!
Note that Harbors can still be built on Lakes, and units may embark there. Of course, since Lakes are by definition landlocked, it doesn't make much sense to Embark units just to cross the lake, instead of say go around it by land.