Here's a little explanation I provided at an earlier date - I hope this thread will help people who are left in a confused state. ;-)

First of all: Wrath of the Norsemen - Full Release is still in development and will remain so for a longer period of time
(I'm not a developer for the full release version but MrNygren et al. are currently really busy with other stuff).
So now to the difference between Wrath of the Norsemen - The Baltic Release and Last Kingdom: Wrath of the Norsemen:
Historically both mods have been developed independently by different teams with Last Kingdom actually having been finished some years ago.
Last Kingdom is based on the books by Bernard Cornwell and tells the story of the Viking invasions into England after the death of Ragnar Lothbrok.
It is therefore not completely historical but nonetheless great fun - note that East Francia is more of a side-faction, the actual main hub of the mod
is the area around the British Isles and the fight Saxons vs. Scandinavians. On the other hand Wrath of the Norsemen - The Baltic Release is a
historical mod set in the Baltic region concentrating mostly on different Slavic, Baltic, Scandinavian and Germanic people - thus its focus is more set
on Viking "politics" at home, the emergence of the Rus and the eastern expansions and christianization of different tribes performed by the East Frankish Kingdom.
It was never really "finished" but is in a more than playable state and, at least for me, great fun. Because it was never finished Wrath of the Norsemen - Full Release
is currently in development trying to finally finish the mod with the old goals of a larger map in mind which had been abandoned to finally release something playable.
You may now ask: So ok, there is Last Kingdom and there is Wrath of the Norsemen, so why is there a mod called: Last Kingdom - Wrath of the Norsemen?
This is because the current developer, MrNygren, of Wrath of the Norsemen - Full Release has also got the permission from the Last Kingdom team to use their
ressources and "continue" developing their mod in the form of a submod which takes stuff from Wrath of the Norsemen and incorporates it into Last Kingdom.
This submod is thus called Last Kingdom - Wrath of the Norsemen and is essentially a bastard of both mods - although the main part is still Last Kingdom.
So in the end you have three choices:
1. You can play Last Kingdom 3.06, which is based on the books by Bernard Cornwell, is set on the British Isles and is essentially finished.
2. You can play Wrath of the Norsemen - The Baltic Release 3.1, which is a historical mod set on the Baltic and Scandinavian regions and is also in a fairly good state.
3. You can play Last Kingdom - Wrath of the Norsemen 3.9, which is a mixture of both although mostly leaning on Last Kingdom.
If you choose Wrath of the Norsemen - The Baltic Release 3.1 you can additionally install my submod Regnum Francorum 1.6.1, which provides a lot of
additional stuff you can read about in the first post of the respective thread. If you want to play as Franks I highly recommend the submod because it is essentially
centered around the faction, thus the name, because it was fairly "underdeveloped" in the base version. I hope this helps a bit - I know it's confusing.