Thugs, pickpockets, and looters who've either outlived their comrades or evaded capture or death at the hands of passing state armies and militias can call themselves proper bandits. Slightly better armed and armored, they can put up a better fight their than their weaker comrades, and will often best groups of farmers on their own.
The big brother of the robber, the bandit is somewhat better equipped, coming to battle with proper maces, the occasional hammer, hand axes (or larger two-handed axes,) swords of generally very poor quality, and hunting bows of various quality if they're lucky. They are on par or better than some recruits, farmers, peasant women, but that's not saying much as these unfortunates generally have even worse weaponry and armor, and bandits will easily fold if facing off against noticeably superior numbers. Generally they are the most numerous criminal units encountered besides the brigand in larger outlaw gangs and may sometimes lead their own groups of 3 - 6 robbers, especially earlier in the game.
This unit can wear plenty of different armours, which happens as bandits have armour from every kingdom (green, blue, brown, red though strangely no yellow or Empire armour, suggesting the Baccus law system is far more effective, or just much more persistent in pursuing local criminals.)