Debian Long Term Support (LTS) is a project to extend the lifetime of all Debian stable releases to (at least) 5 years.
Debian LTS will not be handled by the Debian security team, but by a separate group of volunteers and companies interested in making it a success (with some overlap in people involved).
Thus the Debian LTS team takes over security maintenance of the various releases once the Debian Security team stops its work. The team will thus handle:
Debian 6 “Squeeze” until February 2016
Debian 7 “Wheezy” from February 2016 to May 2018
Debian 8 “Jessie“ from May 2018 to April/May 2020