According to a couple of articles I read, the "chapters" are just another word for expansion packs. Unlike a few MMORPGS I know of, this one actually "has a storyline" and with each chapter new events are revealed. My guess is that they are trying pretty hard to break away from the MMORPG stereotype of more content! More Classes! More items! More Levels! A couple fleeting hours added to something that you thought was already dead! As expected you will get new classes, skills, items, areas, etc, but they are making efforts to make it worthwhile, not just preserve the games shelf life. They will be released about every 6 to 9 months and my guess is they will sell for about 30 bucks a piece.
About that lag issue, I use a cable connection, so during the Alpha/E3 Event I suffered absolutely zero lag. One of my friends played it however, and he has dialup AOL. Regardless of his internet speede running at crap, he said that he had no lag whatsoever, but he did have longer zone loading times (30 secs compared to my 10ish). Once you load an area once though, the loading time dropped to about 2 secs tops -- too fast for me to remember
I hope that answers your questions.