PC users should get the campaign automatically, since Valve has it out as an update through Steam. On the Xbox 360, you'll need to take a trip down to the Xbox Live Marketplace and shell out 560 Microsoft points ($7).
The new campaign includes two maps playable in single-player, co-op, or the competitive Versus mode, which pits human survivors against "super-infected" zombies. Valve has gone on record saying it designed this campaign mainly to deliver a shorter Versus experience, since Versus games can run for well over an hour in the other campaigns.
According to Shacknews, Crash Course also adds a few new maps to the Survival mode, and it brings a couple of important tweaks: multiplayer match-making has been improved, and items now appear at identical places during both rounds of a Versus campaign.
If you haven't sampled Left 4 Dead yet, Valve is selling the PC version for a scant $14.99 until Saturday on Steam.