Welcome to the Aleph Sector 40k (universe) campaign. This is a sector wide "pick up and play" campaign which is largely narrative, but now has a number of specific theatres to cater for different campaign styles.
Sweeping conflict in the Perseus Deeps as the main "campaign system" allows players to simply find an opponent, play the game then record the results with the GM - this generates "points", which the overall faction commander (or the GM if there isn't one), can spend on territory, strategic raids, fortifications or anything else feasible within the campaign. The larger the game - the more points, but the better your faction is doing gives you a bonus to your win. The Perseus Deeps is the main theatre of war in the sector, as it is close to the Cicatrix Maledictum and is key to the control of the sector.
More information here (you don't need to know any of this at all, but the GM (me) works with faction commanders to make best use of their wins, and acts as an advisor, warning against strategically "unwise" moves - though is often ignored!)
Meanwhile in the Hadron Expanse, a more "narrative" driven campaign will take place, allowing players to setup their own missions and agree with the GM what the game is "for", and how the result can be interpreted to drive the story forward.
Finally, (possibly next term), there is the opportunity to run a 40k "Crusade" in the Rifts of Hecate, as small forces delve into the secrets of this newly discovered region.
And if you want background - there is a ton on this blog to delve into. The campaign has been running now since 2001, and the blog since 2006. In this campaign - there really is only war...