Glossary
LTS and ELTS in TYPO3
Long Term Support: the period in which a TYPO3 version receives security fixes as a matter of course.
Why LTS versions exist
TYPO3 appears in major versions, one of which is maintained as LTS at any time. The point is predictability: run an LTS version and you know how long it will receive fixes, and can plan the next update accordingly.
For anyone running a site, that is the most important thing about the whole version scheme. It determines when budget for an update has to be set aside.
The three phases
In the first phase a version is actively developed and maintained. Bugs get fixed, security holes get closed, smaller improvements find their way in.
In the second phase, after regular maintenance ends, security fixes are only available through ELTS for a fee. That is meant as a bridge, not as a permanent state.
In the third phase nothing arrives at all. The installation keeps running, but every hole that becomes known stays open.
What that means in practice
An update is not a one-off task but a recurring item. Plan for it and the effort stays small, because little lies between two states.
Postpone it and you pay twice later: through the ELTS fees in the meantime, and through the larger jump when the update does come due. The effort does not grow linearly, because the changes of several majors pile on top of each other.