What are the differences between skype for business standard and enterprise edition

You can start here: https://technet.microsoft.com/en-ca/library/dn933894.aspx and also https://technet.microsoft.com/en-us/library/gg398254.aspx  which shows deployment options based on company size.

Effectively the difference between Standard and Enterprise comes down to scalability and High Availability.  Standard Edition allows for all features/functionality of Skype within a much smaller server footprint and is better suited for smaller organizations.  However, it doesn't allow for High Availability within a single datacenter. You can pair Standard Edition Servers to provide for Disaster Recovery.  This might be appropriate for smaller orgs that are <5000 users. 

Enterprise Edition uses a larger server footprint and does allow for both High Availability within a single site and Disaster Recovery.  It also also provides for scale and supporting up to 80,000 users per pool.