Skype for Business Server 2015 is a next major release after Lync Server 2013. There is no major architecture level change from the previous release. Skype for Business Server is coming up with many improvements and one new server role “Video Interop Server (VIS)” has been added which will work as an intermediary between third-party video teleconferencing system and Skype for Business Server infrastructure.
Below table gives you a high-level overview of difference between Lync Server 2010, Lync Server 2013 & Skype for Business Server roles.
You can collocate Persistent Chat Server role In Lync Server 2013 & Skype for Business Standard Edition. In Lync Server 2010 A/V Conferencing, Archiving & Monitoring Server roles can be deployed as a stand-alone pool or can be collocated with Front End Server. Skype for Business Server SQL Back End supports all the SQL HA options available such as Mirror, Cluster and Always-On while Lync Server 2013 doesn’t support SQL Always-On.
There are many improvements which has been done in Skype for Business, please stay tuned with me to know more about improvements in detail.