If oyu mean things like game engines (like the quake, quake 2, doom, etc engines, half life 2, etc etc.
Consider the quake engine. It was used in Quake (duh)- but the same "engine" was also used in a number of other games. The engine is the way that game characters and actions are handled. For example, doors are handled by the game engine, as are enemy movement and models, AI, switches and so forth.
This is further illustrated by the fact that most game engines, even for the same genre, refer to the same things with different names or don't even have some things. for example, Duke Nukem 3d was created using the BUILD engine. Since the internal workings of the BUILD engine and how it renders the environment are completely different from quake, these differences are found also in the way effects are created. for Build, creating an effect such as flashing lights or a switch involves the setting of hitag and lotag values on sprites and walls as well as the creation of special "SectorEffector" sprites. Note that it is important to notice also that BUILD itself doesn't recognize most of the effectors used in Duke- Duke nukem itself recognizes a wide assortment and handles them. So while one game might use a Sector Effector sprite with a lotag of 7 as a teleporter, another might use it for something else.
The quake Engine, in contrast, being based on BSP (Binary Space partitioning) can be seen to have a number of concepts from that being used in dealing with the engine. For example, any 3-d Object in quake is a "Brush". Game items, such as ammo, health, enemies, etc, are "Edicts" I haven't mapped much with Quake so I have no idea how it makes various effects such as doors opening and touchplates.
Anyway, long story short- an engine basically let's the game programmer work in higher level concepts- instead of having to work with polygons and vertices, they can think in turns of models, weapons, and so forth. For example, they can deal in concepts such as players, items, objects, and vehicles rather then as a nameless bunch of triangles and vertices.
