What's the name of the type of expansion set "packs" that MTG players can buy at release events?
The format of the pre-release and release party events is a tournament type called 'sealed deck' where you get 6 packs of cards to make a 40 card deck out of.

For recent sets wizards has been printing special "event packs" where one or two of the six boosters you get is custom tailored to a specific colour, colour combo, or deck archetype.  This is to make it easier for new players to build a half-decent deck, since looking at 6 totally random packs and figuring out which two or three colors are best takes some practiced skill!

It comes in a little box with a spindown d20 life counter also.  I think this is probably the product you're talking about.  Some event hosts will sell leftovers after the tournament.  As far as I know this doesn't violate any of their distribution agreements with WOTC.

This is a description of the most recent one:
http://archive.wizards.com/Magic/tcg/events.aspx?x=mtgcom/events/prerelease-facts