In EU2, a province is the relevant quantum of location for most game operations.
All military units are located in a province. Even when they are moving, units are still in the province for all purposes until the very day they arrive in the adjacent province. They are never really "in between" except for the purpose of computing the movement time itself.
There are two main types of province: land provinces, and sea zones. Sea zones are also referred to sometimes as "ocean provinces", or "sea provinces". Unmodified use of province generally means land provinces.
Land units may be in sea zones only as passengers on ships. Naval units may be in land provinces only in a port. Enemy armies must battle if they are both in the same province (and not retreating). Fleets may battle, although they can also "miss" each other.
Sea zones are relatively uncomplicated: they vary only in that some are coastal and some not, and some may occasionally get ice or storms. Also, some sea zones are much smaller than they appear on the map and thus movement into them is twice as fast; these are the fast northern sea zones.
Land provinces have many features, including name, region (continent), terrain, climate, natives, goods it produces (and/or gold mine value), and difficulty for colonization. The values for these features are defined in the file province.csv; some of the most useful fields are summarized in the EU2 Atlas. Land provinces may also contain a settlement. All income is generated in settled provinces.
All provinces are adjacent to a number of other provinces. When provinces are adjacent, movement is possible between them. A land province not adjacent to any other land province is an island. Land provinces may border other land provinces "through" a sea zone, via a strait. Any naval unit in the sea zone associated with a strait blocks the movement of enemy land units across it. This is the only way in which enemy land and sea units directly interact.