Level Transitions
Level transitions allow you transfer the player from one map to another, essentially making two separate levels seem like one larger map.
Why change levels?
The Source Engine, while expansive, still has limits as to the the amount data that can be processed. Breaking up levels also helps split up the work load and compile times during the design process.
Types of level transitions
One way transitions
One way transitions prevent the player from traveling back to previously explored areas after being completed.
Two way transitions
Unlike the one way transition described above, a two way level transition allows a player to travel back and forth between two connected maps.
Creating a seamless transition
To create a seamless transition, parts of one level are included inside the next level being loaded. The exception being "teleportation" style transitions, where the players are transported into an entirely new world or environment. Below is an example from Half-Life 2's d1_trainstation_01, the first level of the game.
Here in the first image you can see the trigger_changelevel in the doorway that loads the next map.
If you zoom out you can see the map were in now ends at the staircase just beyond the door in the first picture. Most of the level lies on the right of the picture here.
Once the player touches the changelevel, the next map is loaded over the old map.
Now the player is in an entirely new map, but from the other side of the door, it appears the old level is still there.
However, if you zoom out, you can see that a new level was loaded and the original level was almost entirely removed. There is no longer the any parts of the level that were in the right from the zoomed out screenshot above, but an entirely new area in the upper left.