1. Navigational Mesh
2. Place Painting

Bots navigation isn't new, but the way it's done with navigational mesh in CS:S is entirely new. Previously, the bots utilized info_nodes to navigate, but the mesh is much more versatile and effective.

The initial nav mesh generation is automatic. Just load up your map in-game, and type bot_add from the console. The game will analyze all the walkable space in yoru custom map, and then figure out the first-encounter points between the two teams. Finally, it will scour all the corners in the map for camping spots. It's a pretty amusing (and amazing) process, but it usually takes a while (up to an hour or more), especially if your map is complex and large.

Hand-Tuning the Nav Mesh

However, the nav mesh that's generated automatically frequently needs to be fine-tuned by hand to be truly effective. There are many places in the automatically generated mesh that makes the bots do stupid things, like jumping off a second story window and jump up and down on stairs like idiots. To infuse some intelligence in these bots, it's essential to edit the navigational mesh manually.

Personally, I see nav editing as the final step in making a good CS:S map. Without it, the players cannot really play your map with bots. The process is almost like mapping, except it's not done inside Hammer. It's entirely accomplished via console commands in-game.

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