Halo 4's progression is a major departure for the series, making many of the items unlocked affect gameplay, rather than pure cosmetics (though there are plenty of those as well). In most other ways, it builds on ideas introduced in Reach. For a detailed review of the progression system, head here.
Rank
Players progress through ranks, dubbed Spartan Ranks, 1-50 through Spartan XP earned during War Games, Spartan Ops and Forge. Spartan points are awarded at each rank, which can be spent unlocking items in the armory. Custom load out items are joined by cosmetic armor, decals and such to customize your Spartan IV with.
XP is also earned by completing Commendations (expanded to ~120) and Challenges.
Specializations
Once SR-50 is reached, players next choose one of eight paths, each of which consists of 10 additional ranks, and which unlock their own set of armor, decals, weapon skins and armor mods. Only one Specialization can be worked on at a time, and must be completed before moving to the next.
At launch two are available, with the rest to be unlocked down the road by 343, unless you have early access via marketing promotions (namely the LE edition of the game or playing prior to Halo 4). Yes, this is confusing and sucks. For more on Specializations,
head here.