Cash apps are businesses that allow money to flow between two parties with low to no transaction fees.   

Paypal was the pioneer in this industry. They allowed free transactions between individuals and charged small fees for transactions related to businesses and that was generally on an honor based system. They have since expanded into credit card processing, invoicing and other services. 

There are many others now.  

1. Cash App

2. Venmo

3. Zelle - (not recommended due to lack of support from the banking network that runs it)