CS-Nation

Covering the future of Counter-Strike
generally, what are the differences between the mod and the retail versions?
As of now there is no difference. With the introduction of Steam all Half-Life content has been localized to a single distribution of files for cross compatibility. Any Half-Life related product will allow you to access the Steam network and recieve everything you need to play the latest versions of all Half-Life content, including Counter-Strike.

In the past, the mod version of CS is the version that you can download if you already own Half Life or any of its various packs and incarnations. The retail version of CS (Half-Life: Counter-Strike) is the copy that you can buy over the counter at your friendly neighborhood department store which sells computer software.

Please note that with the introduction of Steam it is no longer necessary to worry about missing or non-compatible files.