In case you are looking through my collection and think it is a confusing mess (e.g. how can he both "Own" and "Want to Buy" so many games), this is how I organize and tag my collection:
Owned: I use this tag to mark games that I own, I have a PnP copy, or can cobble together (with a deck of cards, for example), that I would be interested in playing when looking for a game from my collection to play. I leave expansions out of this category, unless they are for all intents and purposes their "own game". For example, I play Dominion with expansions mixed and record my plays only under "Dominion", so only Dominion is marked as "Owned". Something like the Korea Power Grid map is effectively its own game (even though it uses parts from the main map/game), so it is marked as "Owned". I use the category in this way so that I can use tools/websites to suggest games to play based on number of players, play length, etc.
Previously Owned: Games listed in the BGG game database that I formerly possessed, but have sold/traded/given away. If I had owned a game but never played it while it was in my possession, it does not get marked as Previously Owned...because it was as if I never owned it in the first place.
For Trade: Games that I no longer want to own, and are available for trade or purchase. Or games that I am *thinking* about getting rid of, usually because they are not getting played.
Want to Buy: Games that I own an official published copy of. I don't include expansions in my "Owned" category, but these would be included here. This category reflects all games/expansions that are part of my "true" collection, and helps me determine what games are eligible for selling or trading.
Want to Play: Games that I have come across that look interesting that I would like to play and/or PnP, and games I own that need to get to the table.
Preordered: I use this tag to mark games that I have PnP'ed (i.e. I technically "own" it, but not the real game).
Wishlist: Games that I am interested in buying (level of interest given using the 1-5 scale for wishlist items).True Collection (Owned Games, Minus Expansions, Minus "For Trade")Owned Games (Including Expansions)Playable CollectionUnplayed GamesNot Yet RatedPlayed "Want to Play"Want to Play