It is a bundled payment gateway+merchant. You don't have to get PCI compliant and you don't need a merchant account (no min transaction volumes, fight chargebacks for you etc).
The down side is they only offer hosted payment pages and not suitable for everything... You need a finite set of goods/services you want to sell (eg monthly plans). You can't automate the process if you're building an online store where your customers can upload items for sale (eg AppStore).
Payment processors refer to their business customers as "merchants".
Entrepreneurs ALSO refer to their payment processors as "merchants".