HP is Hive power, same as Steempower. Blocktrades is the main focal point of Hive, although he's repeatedly claimed there are dozens of experienced devs involved; I think for legal reasons, a lot of them prefer anonymity. Many of the steemit devs (who all quit together during the takeover); have come together to announce their support of Hive.
Communities is like subreddits. You can join South Australia here - trending/hive-190737
When posting, you can choose if you want to post to your blog, or in a community.
Posts in communities don't show up on your blog, so its handy if you have several interests and don't want to bore your Scrabble-fan friends with posts about your dog.
You'd post those in the Dogfan community.
It's a precursor to SMT's; so when they launch, we'll launch OPAL, the South Aussie token; so voting on posts within SA community will award Hive AND Opal.