The Poker Community forum is a loosely moderated forum meant for discussing things like rail calls, big scores (live or online), triple crowns, the PocketFives rankings, poker gossip and so on. In other words, Poker Community is meant to be like the P5s Off Topic community with the only difference being that it's actually about poker-related topics. Poker Community is also meant to be a "catch all" for poker-related topics that don't fit into the other poker forums.
The Poker Sites forum is for topics such as: deposit/withdrawal issues, new feature requests, site software issues, site connectivity issues, support problems, etc. In other words, the Poker Sites forum is for everything related to dealing with the poker sites -- except for the actual strategy involved in playing poker. Just about anything else you do on a poker site or related to a poker site should be posted here.