For clarity purposes, this is my understanding of Nomads, as I got this kind of mission too (I had to kill 42 of them!)
Nomads, as Wikipedia describes them, are "people with no settled home, who moves from place to place as a way of obtaining food, finding pasture for livestock, or otherwise making a living."
In Elite: Dangerous, they can be found as convoys in the instances within Unidentified Signal Sources (which makes them a hard target to find, as it can take a while for an USS to appear, and it might not even have nomads in it!)
They fly the same ships NPC traders tend to fly, and will commonly say phrases such as "are we in civilised space yet?" or be confused as to which star they should of followed. These types of NPCs can be mission targets for certain missions found on bulletin boards of nearby systems.
Commonly, they do not have a wanted status and you will get fined and receive a Criminal bounty if you destroy them. Sometimes, however, a Nomad can have a wanted status on it, oddly. killing any Nomads (marked as "Mission Target") Will contribute to the mission (as long as its within the system specified)
Just like normal NPCs, they can have all sorts of cargo in them, including rare commodities from nearby systems and unique cargo such as 'Assault Plans' and 'Trinkets of Hidden Fortune' (which is worth only around 1000 credits, however it is a funny novelty cargo to have).
I haven't completed the mission I got yet, but I found it in Liaedin (Fortress Anderton) and I had to kill 42 of these hard-to-find NPCs within 3 days, and it only paid just under 1,000,000 credits, but did have a high reputation impact (and medium influence impact). For me, it took half a day or so to get 33 kills. (I'll add a picture of the mission sometime tomorrow.)
In my opinion, only take these types of missions if you think you would like searching within Unidentified Signal Sources, and/or role-playing as a mercenary who has been hired by Donald Trump to kill space Mexicans/immigrants.
Edit: too lazy to edit images, so here are screenshots of interfaces related to the mission I got.
http://imgur.com/a/Fzbvt