What are the difrrents between ELK 6 And 7?

Hi guys, we are using ELK 6.8.1 (Elastic, Logstash, Kibana) open source version.
whem i'm doing yum update we are getting 6.8.2.
Version 7 is free ? no free?
i will be happy if someone can explain why do we have version 6 and 7?