I'd be very surprised if someone went their role-playing career without encountering a bandit. These guys are everywhere. And not only are they an easy source of low level baddies in campaigns, they say something about the location they are found in. From brazen daylight robberies or late night pick-pocketing, there are a lot of schemes these guys can be used for. And it is in the hopes of arriving at more ideas to use these baddies that we'll explore them today.
A group of bandits should have some sort of money making scheme. Pick-pocketing the rich, attacking caravans, and extorting high ranked people for favours are all examples of things that they can be trying to do. However, this element is very important for your group of bandits as it will determine their motivations and often their skill level. A group of assassins in a big city will not be comparable to a bunch of highway robbers far from civilization. It will also affect how exactly the party will be hindered by them and in what manner. Change the scheme, and you change the entire situation with these guys.
A few thoughts about the organization of the bandits is a good idea. Is it a centralized structure with some outside groups responding and doing as they are told? Is this a small operation so there is only one level of management close to where the action is? Or is it a large, decentralized organization that occasionally gets some orders but has a lot of leeway as well. What this does, besides helping us understand how the group might act and whether they can get help from another office, is let us think about the heads. How many levels of organization are there? Is there the regular bandits at the bottom and one level 2 leader above them? Or is there an evil level 18 wizard leading a level of level 10 enforcers who have another 3 levels below them? With that kind of organization, they could get into conspiracy theory territory.
Of course, bandits could be convenient henchmen for something bigger. They could be a front set up by a noble to take care of business that can't be dealt with legally. The operation could've been taken over by mindflayers or vampires. Do they know? Maybe yes, maybe no. Regardless there is a layering of threats in this case. Making sure these layers make sense together is important. Also, the bandits could eventually become the henchmen of your characters. Perhaps they are competing with your players to steal something.
Oh, it turns out they aren't bandits. They are an invading army looting or something. Kind of a copout? Yeah, but it can work. The important part here is that banditry is an occupation, and people can use it as a front for something else.
How do these bandits fight? By the D&D 5th edition book, they'll have a scimitar and a crossbow. However, we can play with this a lot and in fact likely will need to. If this is all in a city, you wouldn't expect your bandits to walk past guards wearing weapons in a slum. Instead they might be wearing concealed knives. Likewise, they don't all have to be the same bandits. One might use a whip and short sword, one might hand back with a bow and help their wizard, and another two might try to hold the enemy in a desirable location with shields and swords. Perhaps they don't actually fight. Instead they lay quick ambushes, attack, and then disappear. Or maybe they are experienced and rely on a combination of martial prowess and magic. Not every group in the bandit organization needs to have the same outfit either. In general, I'd recommend treating the entries in the Monster Manual for bandits as a starting point.