There's a series of timelines superseding one another. In the "true" original Trek timeline...
- The Eugenics Wars were open warfare, and happened between 1992 and 1996.
- World War III cost 37 million lives in the early 21st century.
- Warp engines were discovered by Zefram Cochrane of Alpha Centauri in the mid 21st century, shortly after first contact with Earth.
- The USS Enterprise NCC-1701 was the first starship of that name. Robert April was its first captain, Chris Pike its second, and James Kirk its third.
- The events of TOS, TAS, and movies I-IV happened basically as depicted, in the early 23rd century
- Romulan culture is as described by Diane Duane. Klingon culture is as described by John M. Ford.
Basically, this is Star Trek as one would know it from watching the original series and films, and reading certain key early novels plus Bjo Trimble's Concordance, the Goldsteins' Spaceflight Chronology, and a smattering of other Treknical lit... the state of things up through about 1987.
THEN something happened (maybe some side-effect of the careless time travel in ST IV?), giving us the timeline seen in TNG. (Recall among other things that it was the final episode of TNG's first season that provided the Gregorian date on which Trek timelines have been hung ever since.) In this reality...
- World War III was much more devastating than it had originally been, costing 600 million lives and leaving parts of Earth in a "post-atomic horror" as late as 2079
- TOS, TAS, etc. happened roughly 60 years later than they had before
- Klingons, Romulans, and the entire 24th century are as depicted in TNG and DS9
- Kirk died as depicted in ST VII [Generations]
THEN we get a new cascade of timeline changes (let's call it a combination of the effects of ST VIII [First Contact] and ST:Voyager's "Future's End"). In this one...
- The Eugenics Wars were fought covertly
- World War III was over by 2053
- Zefram Cochrane was from Earth, and discovered warp technology in 2063, coinciding with humanity's first contact with Vulcans
- Events of the 2150s were (substantially) as depicted in ST: Enterprise, including the existence of the NX-01, earlier-than-previous contacts with Klingons, Ferengi, and Borg, and somewhat altered history surrounding the Romulan War and the founding of the Federation
- The 2230s looked as depicted in the early scenes of ST XI (pre-Nero)
- The events of ST IX [Insurrection] and ST X [Nemesis] took place in the late 2370s
THEN something else happens c. 2380 (I'm really not sure what to pin this on; open to suggestions!) to splinter the timeline into at least two main VARIANTS, including...
- The reality depicted in the post-Nemesis Trek novels, including (e.g.) the final end of the Borg as depicted in the "Destiny" trilogy and subsequent events
...and, separate and incompatible with that...
- The reality seen in the framing scenes in ST XI, in which Romulus is destroyed; this also leads into the early 25th century history found in ST Online
...AND of course, as an offshoot of the second of those, we get...
- The main reality depicted in ST XI, in which (after changes in the 2230s) the Enterprise NCC-1701 is built later than before, but Jim Kirk takes command earlier than before, in 2258, and oh, yeah, the planet Vulcan is destroyed
So by my count that's (at least) six main timelines involved, just to account for the variations familiar to us from "canon" Trek. Unfortunately for me my personal favorite of these is the first, but it's also the most remote from anything being written or produced these days or likely ever to be seen in the future.