Can I use an AT&T iphone on Verizon?

My Aunt gave me her old iphone 4s as a gift, but it was used on an AT&T contract. If I took the AT&T SIM card out of the Iphone, would I be able to replace it with a different one and use the Iphone on a Verizon contract?

No sorry even if you unlock the phone you can't use it on a Verizon network. This is because Verizon is very strict about their phones and sadly you can't use any phone on a verizon network unless it is made by verizon.

You'd have to get the phone activated on Verizon.

Verizon is a CDMA network which doesn't use SIMs. Also the phone would not be unlocked.