The following is a basic guide for prospective modelers with little to no experience in video game modeling who would like to learn just what it is we mean when we say "low-poly" or "UV-mapped".
When modeling for our game, we need to keep a close eye on polygon usage, which is how many polygons, or triangles, make of any given model or scene. Lower polygon counts are better for game performance, and should be implemented wherever possible.
In the above example of the spheres, the top version is what one should aim for. Polygon counts should vary depending on the scale of the object and how closely the object can be seen in-game. A tiny object located far from the view of the player should be fairly low-poly, while a larger object that can be seen up close can afford to be higher-poly.
Objects heavy on curves and which can be seen up close, like these vents, should have enough polygons so that it appears smooth up close, as seen on the right, but not too many as seen on the left.
Simpler objects that are present in high quantities, like these chain links, should be of a reasonably low poly count.
Tubing should have as few sides as possible without making it look too blocky and depending on how close you can see it. In this example from a crane platform railing, the top tube has 8 sides and relatively few divisions, but enough to make a decent curve. The bottom tube is overkill.
Extremely simple objects like these cylinders, like any other objects, should not have more sides and divisions than necessary. On a plain cylinder, for example, there is no reason to have 10+ divisions along the height or multiple rings of edges on the top as does the one on the left.
Extremely complicated objects that can be inspected up close and are fairly large, like this winch, can often have fairly high poly counts, but their individual parts should still use as few as possible, or strike a balance between poly count and smoothness upon close inspection. In the case of Titanic, this is common as one of the goals of our game is to allow people to inspect as many aspects of the ship as possible.
Note the details of this mooring bitt model. In areas where it may be easier to see "hard edges" around curves, it's higher-poly, while other areas are lower-poly.
When modeling objects for the game, you can use quads (polygons with 4 vertices) or triangles (polygons with 3 vertices). However, the final model should always be triangulated before being sent off to be placed in-game. The above example of a bell shows a model made of quads on the left, and the same model triangulated on the right.
Once the models are done, they need to be UV-mapped.