Short On-Point Description: 

In brief, RDP provides remote display and input capabilities over network connections for Windows-based applications running on a server. At the same time, VPS is a virtual machine that you can use to host your data, and since VPS runs its OS, you can get a Windows or Linux VPS server.

Main Conclusion:

Our VPS servers are servers that have Remote Desktop Protocol access enabled in them, and that's why we refer to our Servers as "RDP" due to their way of access and also due to the popularity of the RDP term being used between our existing and new users to refer to them.