The concept of tagging isn't new (YouTube, this very website) and Steam offers a few ways to take advantage of Steam Tags. You can browse popular tags, your friends' tags, and your own tags. Based on the games you've already played, tags are also recommended for you. Tags will help generate filtering controls to give users the games they are looking for; Steam has grown larger over the years, and as a result, "discovery" of new games has become more difficult for the end user. This should help aid discovery of relevant, interesting games for players.
Steam's adding tags so late to the game that the crowd-sourcing effort is likely a necessity: Tags can't really be automated accurately (we should know -- we've had to manually retag 1000 articles before), and now that Steam has in excess of 3000 games in its catalog, a dev-only tagging effort would be slow on the uptake. That's where players come in.
You can read more about Steam tags and how to start using them over here: http://store.steampowered.com/tag
You can browse popular tags here: http://store.steampowered.com/tag/browse/#global_19
- Paige "dino pillow" Spears.