To understand the Azure Marketplace let’s start with the Virtual Machine Gallery and a tale of two portals.
The Virtual Machine Gallery allows you to quickly and easily deploy a Virtual Machine running your desired operating system (OS) such as Windows Server or a Linux distro such as Ubuntu, Suse etc. It also allows you to deploy a Virtual Machine pre-configured with a workload such as Visual Studio, SQL Server, SharePoint (trial), BizTalk, Puppet and more.
The key point here is that is makes it easy to deploy a single pre-configured Virtual Machine image.
What powers this? Something called Service Management API which internally uses “RDFE” which stands for RedDog Front End (RedDog being the code name for Azure back in the day).