The Northmen who dwelt in Greenwood the Great and other parts of Rhovanion were friendly to the Dúnedain and were, after the Dúnedain, some of the most noble Men in Arda. They were known as Middle Men by the Dúnedain, and were believed to have been descended from the same group of Men as the Edain, the Atanatári. The only difference was that they didn't cross the Blue Mountains (Ered Luin) into Beleriand, and therefore did not journey to Númenor. The result of them not participating in the War against Morgoth was their considerably shorter lifespan; the lifespan of the Dúnedain being lengthened by the Valar after the War of Wrath. They were important allies of Gondor and served as a buffer against the Easterlings and other foes. For a time many of them even became subjects of Gondor, as the realm extended beyond the river Anduin.
East of Greenwood the Great was the kingdom of Rhovanion, and this became the most important nation of the Northmen. In the fourteenth century of the Third Age, King Rómendacil II of Gondor sent his son Valacar as an ambassador to Vidugavia, the so-called "King of Rhovanion". Valacar loved Rhovanion and the daughter of the King, Vidumavi. He married her and she bore him a son whom she called Vinitharya in her mother tongue. In Gondor, Vinitharya became known as King Eldacar and caused a civil war- the Kin-strife- because of his mixed Dúnedain/Northmen blood.
Shortly after the death of Vidugavia, a small group of Northmen moved to the vales of the Anduin between the Gladden Fields and the Mirkwood, probably to flee from the Wainriders, who had enslaved much of the people of Rhovanion. After the evil Kingdom of Angmar was defeated by Gondor and the remains of Arnor in TA 1977, these Northmen moved north and began to call themselves the Éothéod. They were skilled horse breeders and horsemen. In TA 2510 they responded to a plea of help from the trapped Gondorian army at the Field of Celebrant. After they helped Gondor win this important victory, they were rewarded the province of Calenardhon and became known as the Rohirrim. The Men of Rohan who fought on foot were called 'Yeomen'.
The Men of Dale and Esgaroth were also counted under the Northmen, as were the woodsmen of Mirkwood. Most of Rhovanion had been depopulated after the wars of the Easterlings and the Great Plague. Their range was extremely wide; the Men of the North ranged to the western eaves of Mirkwood to the southern slopes of the Grey Mountains and from the Long Lake to Dorwinion on the Sea of Rhun.
In the Hobbit, these men carved out a potentially precarious living in the eaves of Mirkwood, living as hunters, loggers, trappers, and fishermen on the Anduin and its tributaries. They also lived in constant fear of attack from Dol Guldur.
The larger, more established kingdoms of the North (such as Dale and Esgaroth) would have at least some semblance of a regular standing army. These would have mostly consisted of a force of dismounted men-at-arms, armed with a sword and a wooden shield. The Men of the North might have received some Gondorian help in metalworking, along with their Dwarven allies nearby. A militia or town guard would have also been used extensively, for the majority of the humans in the north were farmers and thus needed their full attention kept on the all-important harvest.
Further up the military hierarchy would be a force of axemen, for many of the woodcutters and loggers in Mirkwood would have learned to use the axe from adolescence.
A force of mounted knights and light cavalry would probably have been available for scouting and other duties, for the terrain from Dale to Dorwinion is mostly flat and featureless, except for a few trees, shrubs, and ponds scattered randomly.
By contrast, the rural, less established kingdom of Rohan trains almost every legitimate soldier it has to fight from horseback, while the infantry division is left to be composed of not soldeirs but, as Gamling states in The Two Towers, "every villager able to wield as sword".
Being human means that the Northmen spoke the Common Speech. However, they also have native languages. For example, the Rohirrim spoke Rohirric, for which Tolkien appears to have been inspired by Old English, seeing names like "Eowyn" and "Thengel". Whereas the Bardings and Wood-men speak a similar language to the men of Rohan in terms of gross language group, as seen in Old Norse names (akin to Old English and Norse being in the same language group), i.e. the name "Brand" translating to "torch". The language was not similar to Dwarvish; rather the Dwarf names appearing in The Hobbit and The Lord of the Rings are Dwarf names given in the language of the men of Dale, not the native Dwarf language. They never shared languages with the Elves or with their Western, Southern, and Eastern counterparts; other than Westron.
The People of Middle-earth