Micro SIMs are a type of SIM card used predominantly in the latest iPhones (4 onwards). They work the same way as standard SIM cards as they allow the phone to be identified by the network which means you can make calls, texts and use the Internet. As with standard SIM cards you’re also able to save data onto a micro SIM.
If you are changing phones to one with a micro SIM then you will need to swap SIM cards as your normal SIM won’t fit into your new phone. If you’re a Pay Monthly customer then a new micro SIM card will come with the phone. If you own a phone but don’t have the correct SIM for it then contact your network to request one.