In Short

Wallet is the new name for Passbook in iOS 9. Just like it did back when it was first introduced back in 2012, wallet acts as a single repository for all the movie and sports tickets, loyalty and payment cards, boarding passes, coupons, and other passes generated by all the compatible apps on your iPhone, iPod touch, and Apple Watch.

Since 2014, it's also served as the home of Apple Pay, collecting all your credit and debit cards as well. With iOS 9, Wallet will also gain support for store and loyalty cards.