Each server you're using for Skype for Business, provides 1 or more server roles. You don't need all server roles - only deploy the roles which you need.F or most server roles, you can deploy pools so as to provide scalability and reliability. Hardware and/or DNS load balancers are required to spread resources around a pool.
Standard Edition Server is for smaller organisations, or pilot projects in larger ones - you can run Skype For Business entirely on one server including the necessery databases along with IM, presence, conferencing, enterprise voice.
Front End and Back End servers work together. Back end servers provide the SQL databases used by Skype for Business, but do not run and Skype For Business functions SQL mirroring, clustering or always-on is recommended at the enterprise level. The front end servers are the core server roles, providing SfB functions.
Edge Servers allow communication and collaboration to users outside the organisation's firewall. This can include offsite works, federated users and outside users included in conferences. There is also connectivity to public IM such as Skype.
Mediation Servers are used for translation between internal SfB infrastructure and a IP PBX, PSTN Gateway or a SIP trunk. This can be a standalone pool or run on the front end server.
A Director server does not hold any services or account and most useful in enhancing security by authenticating requests to prevent DOS attacks.
Persistent Chat servers and provide and provides multi party and topic-based conversations which persist over time. The back end server stores, the chat data and information about categories and rooms.
Video Interoperability Server - this can allow you to integrate to 3rd party video conferencing. It's a standalone server role. Acts as an intermediary between Skype for Business, and Cisco conferencing systems.