Just a Steam installer, that's it. A DVD with a whopping 8 megabytes of data. In other words, the retail version is a Steam key bundle with some useless plastic that'll go straight in the bin (no reason to keep it).
Yet, despite this waste of plastic, manufacturing and transport, the retail version is almost 20% cheaper than buy on Steam, at least here in Sweden.
Retail price is around 399 sek which convert to roughly €42, on Steam it costs €59.99.