What will happen if I use Wi-Fi and 4G LTE at the same time?

I'm buying my first smartphone and its going to be a iPhone 5.So I I know you can use Wi-Fi instead of data network but How do I do that? Do I need to turn of Data Network?

Also if I have LTE and Wi-Fi on. Which will the phone use?

If you have 4G running, but your in range for Wi-Fi, it will go for Wi-Fi. But if your not in range but its still on and your running 4G, all it'll do is use a lot of battery. If Wi-Fi is available, use it.It'll use less battery

The best way to explain this is:

When you buy the iPhone 5 and connect it to your home Wi-Fi, everytime you walk into your house, it will switch automatically to the Wi-Fi. It will keep the LTE on for phone call pursposes only. It will not charge you data when you are on your Wi-Fi.

When the phone loses the Wi-Fi signal, ie, you leave your home, the iPhone will automatically switch to the LTE and you will be using that for the data.