Is iPhone 5 only 4G or is it 4G and 3G?

I'm not entirely sure what 4G is. When I get my iPhone 5 my mom will add me to her plan (wich also includes my dad). They have 3G but have an iPhone 4.So when I get mine will it be 4G?

Added (1). But do I have to pay extra for it or will it just be there if there's a 4g signal?

IPhone 5 = LTE/4G which mean it has 3G and 4G. The G stands for generations. IPhone 4 has a 3G network so the download/upload speed will be slow. But iPhone 5 has an LTE chip which is faster than Wi-Fi. You get speed like 30mpbs + (depends on the carrier).

You don't pay extra for 4G. It's just better service.

4G/3G are just forms of mobile data (internet on the go). The number just corresponds to the speed.

All 4G phones can also connect to 3G and below (in case the 4G signal isn't strong enough)