There are two main systems used for rainwater capture. The dry system and wet system. See the animations below:
The Dry System
The Dry system is easy to understand. The water falls on the roof and runs down the gutter into the downpipe and directly into the tank. It's called a dry system, beacuse when the rain stops there is no water in the pipe. This is a good system when you have a large roof area, running into one or two downpipes that you can direct to the tank. Remember to get the tank height right so you have fall to your tank.
The Wet System
Water is always level and this system utilises this fact. As the water fills the pipe, it rises equally and as the tank is lower, the water then enters the tank. This system is good for capturing your whole of house or multiple down pipes. It called a wet system because when the rain finishes, the water will sit in the pipe ready for the next rain event. It is important in a wet system that you stop leaves entering, using gutter guard or similar, as the leaves can rot in the wet system and discolour your water. Most new homes built under the NSW BAISX scheme use the wet system.