If you aren't familiar with an ATB type of battle, play the first 10 minutes of Final Fantasy 6. They're simply a system where agility slowly builds a timer represented by a gauge. When the gauge fills and the timer hits 0, players and enemies take actions. It is not that radically different from classic turn-based systems, but is stereotypically a little slower.
I think I understand ATB now, it seems really strange compared to Turn-Based though,
I'm surprised you haven't played any games with an ATB system, before. Final Fantasy has done it for almost every game since the 4th one (though you couldn't see the guage fill until the 5th), and some people have written scripts that do the same thing in modern RPGMs.
The point of ATB was to create more dynamic battles and keep you on your toes. Because while you're sitting there deciding on your character's attack, the enemies can still attack. Assuming you have "Wait" turned off. If it's turned on, everyone patiently waits for you to choose your attack. And it looks like it's on by default, unless there's an option in the database to turn it off by default. Personally I prefer it off, because battles go a lot faster that way.
Also, because of the natue of ATB, you don't input your entire team's attacks at the same time. Only when their individual turns come up. Oh, and regardless of whether "Wait" is on or off, you can also choose attacks while attack animations are playing. So the fact that you have to wait for a person's guage to fill up is balanced out by the fact that you don't have to wait for anything else.
