iOS 11.2 brought a new feature called Apple Pay Cash that allows you to send and receive
money using iMessage app on your iPhone. The process is simple, fast and secured
as claimed by Apple.

The money sent using Apple Pay Cash is simple as sending a message. Just link your debit
or credit card and start sending money on iMessage with a touch of a finger or
by asking Siri.

The money received on iMessage app is stored on Apple Pay Cash and can be instantly used to make purchases in appstore, web, online sites and can be even transferred in Bank accounts.

Apple Pay is easy and works with the Apple devices you use every day. You can make secure purchases in stores, in apps, and on the web. And now you can send and receive
money from friends and family right in Messages. Apple Pay is even simpler than
using your physical card, and safer too.

Send and Receive Apple Pay Cash on iMessage app on iPhone

Step 1: Launch iMessage app
Step 2: Select a person who you want to pay (iMessage ID)
Step 3: Tapand Tap
Step 4: Enter the amount that you want to send
Step 5: Confirm your transaction with Touch ID and Face ID in case of iPhone X
Step 6: Tap Send and it will instantly send money to the receiver
Receiver can see their amount received in Pay Cash section in the wallet app
Check out this How To video from Apple for more details: