Play by post is basically playing by text. The GM and players will make posts (usually in the in-game chat, but sometimes on the game's forum page) describing what's happening in the world and their reactions to it. It can be handy for someone who never really has a set time they can play, as they can just pop in and write a post whenever they have the time. However, the game also tends to move *extremely* slow due to the large gaps of time between making a post and getting a reply. Unless the GM and players are on at the same time, expect to wait about a day to a couple days between replies, depending on everyone's schedule. Also, the brevity of the posts and lack of time for getting too into the mechanics means that a lot of the time the GM will have to cut out most of the actual game mechanics in favor of simply rolling to see if you succeed or fail an entire encounter or scene. Very good if all you want is the roleplaying experience. Not so good if you want to learn the actual mechanics of the game. As for your other questions, I would try to find a GM willing to communicate with you on a regular basis as to what time you'd be available to play, and try and gather a small group of other players that are ok with that type of schedule. If you want to be a player, that's going to be the best bet. If you want to be a GM, then obviously you can set your own schedule and simply inform everyone of it before they start playing.
