7-day and 30-day campaigns relate to the time before the end of campaign rewards are handed out. 7 day campaigns have lower end of campaign rewards than the 30 day.

You can only "home" in one campaign where your AP counts towards end of campaign rewards or emperorship. You can "guest" in other campaigns and earn AP, but it won't count for end of campaign rewards. You can pay AP or gold to swap your home campaign in the middle of the campaign or there should be an option to leave at the end of the campaign for free (this might be a bit different during MYM since ZOS usually lets people swap campaigns).

If you exit the campaign to PVE, you retain all your earned AP on the leaderboard/end of campaign rewards. You'll have to queue up like usual if you want to return to Cyrodiil.

The big thing to note with the Alliance locked campaign is that it is locked to characters in one alliance, determined by the alliance of the first character you enter it for five minutes after the start of the campaign. If you want to swap alliances on that campaign, you need to wait for the current campaign to end. In Standard campaigns, you can play for any and every alliance.

The 7 day campaign's are just around for MYM.

We only get 30 day campaigns outside the PVP event.