Nomads are the people who for whatever reason do not put down roots and settle in one place. They don't have cities and in general will have lives based around some kind of herd animal. Another generalization is that in games they tend to be the low-tech culture. Need some people just above savages? Throw some nomads in. Now I don't like that so lets see some ways that nomads can be included where they are different from the normal view.
A nomad culture that is ahead of everyone could be the elves. Why would this be you ask? Well the elves have this in touch with nature thing going on and no matter what you do if you have a group of intelligent beings live in one area to long it will affect the land around them. To better describe this I will go into how I plan to make elves in my world I am making.
In the beginning after the Gods created all creatures the elves became powerful. Their culture was steeped in magic and natures way but instead of following this they sought out more power and with the dwarves they developed great feats of technology. Then The Disaster Came. Their lands where destroyed and only elves that where away survived. Following this despite keeping much of their technology they no longer stay in one place for long, fearing a time when once again they stop listening to natures way. Careful to never leave a trace behind they have bent most of their magic and technology into highly portable forms which will last for long after the creator is long dead and being elves this is indeed a long time.
The elves of my world will be the leaders in technology and magic but because of their fear caused by the loss of their homelands will never settle down. In this they are nomads by choice unlike some cultures that may be forced to be so. The elves will in be one of the most stable cultures so their mastery of technology though constantly being refined could be eclipsed by humans because they have very little innovation.
You can always go for the cursed culture. A group of nomads who constantly move not because they want to but because they where cursed so that they can't not. Maybe in the past they offended a god of chaos and where cursed so that if they ever settle for more then a couple seasons in one place the land would warp and creatures would mutate. Monsters would spawn or be drawn from miles around and the settlement would be in constant danger. Of course with the curse there should be some limit such as there has to be say 10 or more of them in settled so you don't have nations paying a single person to live in their enemy's home city.
That is not the only curse you could do though it could be used to explain where monster came from in the past. It also does not have to start out as a curse either. Maybe their god blessed them but when they wanted to settle the blessing backfired. Having it so that any of your buildings are able to be taken down within moments would be an amazing thing for nomads but if it applied to all buildings and not just their tents then you could end up with their houses falling down like a house of cards. There would be no way for them to make a city if anytime an earthquake happened or whatever all the buildings fell over.
This could not only be an interesting way to have nomads but would also be a possible adventure hook. If you let your players change the world then being able to uncurse a whole nation of people would be a feather in their caps. Freeing a town of an evil curse is considered quite the heroic thing to do at mid levels so this would be the high level version of it.
A race of carnivores could be a possible nomad culture. They can only eat meat and they prefer a certain type so they raise giant herds of the animals and travel around to make sure they have enough food. The reason humans settled down in the real world was the discovery of farming so they no longer had to keep moving around. If we where restricted to meat that step could not have happened.
The race could be as advanced or savage as you want and in fact don't even have to be fully nomadic. If you want then they could have cities in a few places for things like trading with the bulk of their people tending the herds. Even if fully nomadic it doesn't stop them from having all kinds of stuff. Even in real life it would be possible to have metal working and such for the softer metals. Once you add in magic then any kind of limits go out the door really. You could have them with forges in the back of wagons and not end up burning the wagon if they just put a ward against fire.
Even if you limit things like being able to work metal all they would need is one permanent city. Just have the city in the middle of their territory and have it be where all of their technology gets made. It could also be where say, the ruler lives or whatever else you want to have in place for easy access by the players.
The people could have magical reasons for staying nomads. Depending on how your worlds magic works it might be that they are traveling between magical hot spots that change through the seasons. A highly magical society could have all the comforts of a settled one with a little bit of effort. A bag of holding is easy enough to carry so why not make something similar only the entrance is a door and it leads into a room. A bottle of air and your good. Just add some levitating carts to provide a smooth ride and your good.
Maybe the reason they don't settle down is that all of their stuff uses so much magical energy unless they follow the moving hot spot around it would fail. Having to constantly move where your front door lets out seems a small price to pay to live in luxury.
In Advanced Labyrinth Lord a first level Magic User can cast unseen servant which lasts quite a while and a 3rd level Magic User can cast continual light which if they spend enough time at it means a city never need worry about lights again. In a culture where everything is done by magic there should be enough low level Magic Users running around that no one would need to ever physically clean anything or use fire for light ever again.
To finish with this topic I will say that I am not against nomads just being nomads. I just would like there to be a better reason then "we needed some savages" or whatever. Give them a reason to be there and a reason that in a world full of magic they still do what they do. Even if the players never find out why a culture does what it does knowing it as the DM will let you make the world that much better. If the worlds backbone is strong it means you can let the players do as they want in it and avoid unseemly plot holes.