Welcome to the Combine, and lets see if I can help answer some of your questions in the order you have them.
Holoboards - are just some people way of saying the forum pages here. Be it Combine webpages or Faction webpages.
Darkness or Darkness messages - Darkness in game usually means an Admin Faction, They hold no ties to anyone and don't like to be bothered.
Darkness controlled planets are not shielded and no one is given build or arrest privs on them, so they are usually safe for people to go.
Darkness messages - are the same as Pm's or private messages on other websites. They can be used to contact other players besides Irc and the forums. (( the envelope icon in the upper tool bar is the Darkness Message button, if you have a DM, it will flash yellow. ))
Now for the other Questions.
HP is the amount of Hit Points your character currently has. You gain more of these by leveling up or if you happen to get injuried they raise slowly over time or can be brought up faster with using the medical supplies.
XP is the amount of Experience points you currently have and need to level up your character giving it better skill points in the fields you are interested in. Be it piloting ships, R&D and so forth.
To get XP, you do things in game. (( Walking around the city, piloting ships, things like that. ))
CP is Combine Points, These are little reward points that are earned for helping out SWC. Be it from voting on the Websites, helping make SWC better (( helping with Descriptions, things like that.))
These points can be used to purchase things in game, like Ships, Vehicles, Droids as well as adding Custom images as well.