What will happen if i switch sim cards?

So my iphone 4s won't turn on and i need to get a new one. If i was to take the old sim card out and put it into the new iphone would i get any of the contacts i had? What does the sim card do?

I don't think so, the sim card in my iPhone 3GS had only my phone number and service provider information. Best way to keep your stuff is periodically back up your iPhone information that way you always have your stuff. If it has been backed up, plug the new iPhone in to iTunes and sync it and hope you get your stuff!

If you have saved your contacts on the SIM card then they will appear on the new phone. The SIM card keeps data needed by your phone to connect to the service provider or phone company. For one, it stores your contact number so even if you change phones, if you use the same SIM card, you use the same phone number. Storing contacts is a standard feature in SIM cards but is optional depending on the setting of the phone.