Each of the six "race packs" (which also include the game itself) provide additional goodies for one of the game's six factions, are digital download-only offerings, and will only be available until the game releases exclusively on Windows PC on March 1, 2011.
Each race pack retails for $29.99, and provides players with four unique abilities and "bonus Wargear items" for the race associated with the pack, and can be used during the single-player campaign. That said, THQ notes that the bonus content included in each pack will be made available for purchase separately on Steam after the game's release "for a nominal fee."
The collector's edition, which can only be purchased by pre-ordering through the THQ.com store, includes all six "race packs," as well as "an exclusive poster, six race art cards and a high-quality collector's box." The collector's edition will run you $39.99, however, it's worth mentioning that the THQ.com pre-order page wasn't yet live at the time of this post. It's also worth noting that versions purchased through THQ.com will still utilize Steamworks.
The Space Marine, Chaos Space Marine, Eldar and Imperial Guard "race packs" are available for individual purchase on both Steam and THQ.com, but the Ork Race Pack is a Steam exclusive, and the Tyranid Race Pack is exclusive to THQ.com. Just something to be aware of that if there's a specific pack that you're interested in.