Chaos Chosen
Chosen in 40K
A Chosen is a term coined up by Chaos Space Marine Warbands that describes a particularly powerful member of a Warband. A Chosen comes in many forms. They may be very experienced warriors that were chosen by their Chaos Lords to lead squads of Chaos Marines into combat, some are literally Chosen by the Chaos Gods (or a single patron) and given "gifts", others may be powerful Champions that are lesser Chaos Lords serving a more powerful individual. Whenever the case, all Chosen are characterised by having unholy bling, millennia of hard won experience (stretching back from the days of the Great Crusade to present days 40K), mutations that are actually useful and sweet wargear.
Chaos Chosen usually start as normal Chaos Marines that slowly and steadily advance the hierarchical ladder of their Warband. They may become members of a Chaos Lord's (or any other distinguished Chaos Commander's) bodyguard squad, eventually becoming Chaos Lords themselves if they keep up with the villainous heroism/Just as Planned ploys and piousness/satisfying the Dark Gods/patron god (even going so far as becoming Daemon Princes or that-whose-name-we-will-not-utter if they screw it royally up). Hell, it's actually possible for any Chaos Marine (even a Possessed, a Obliterator/Mutilator, a Hellbrute, you name it) to become one as long as they become better at what they do and show more interest in other forms of warfare (how is this possible in the case of the latter four? dunno).
Interestingly, Chaos Chosen that want to gain Terminator Armour need to gain it in different manners as opposed to their Imperial counterparts. Among Chaos Marines, Terminator Armours are limited (unless they have ties with the Dark Mechanicus, and even then these guys are no different from the Adeptus Mechanicus in term of hoarding stuff, so they rarely give the Spiky Boyz any newly made bling unless they get something valuable), thus in order for a Chosen to obtain one, they need to take it away from another Chosen either via heavily ritualised combat (aka: the manly duel to the dead), or less warrior like (good old backstabbing or "unfortunate turn of events").
Chosen in Fantasy Battles
The Chosen in Warhammer Fantasy Battle function the same way as their younger 40K counterparts, yet instead of edgy Chaos Marines, we have manly barbarians in heavy armours and so much muscle that they would make Arnie look bleak.
The more favour they gain and show off their warrior capabilities (being able to slay something really big or prove their tactical capabilities to command), the higher they climb the hierarchical ladder up until they become Chaos Lords themselves, and from there they can either become Daemon Princes, or Chaos Sp...you know what.