Humans were the second beings created by the supreme God, Ilúvatar. Because they awoke at the start of the First Age, at the first rising of the Sun (FA 1), while the elves awoke three Ages before them, they are called the "Afterborn" (Quenya Atani, Sindarin: Edain) by the Elves. Humans bear the gift of men, and therefore they age and die when their time comes, and are susceptible to illness and disease. Elves are immortal, in the sense that they are not susceptible to aging and disease.
The Arnorians are humans that live in the northern kingdom of Arnor. It is referred to the ones that are the Dúnedain, blessed with long life, and descendants of the the Númenóreans who survived the fall of their island kingdom (Númenor) and came to Eriador in Middle-earth, led at first by Elendil and his sons.
The Gondorians are humans that live in the region of Gondor. They were descended from the Númenórean colonists who settled there throughout most of the Second Age and their noble kin who escaped the destruction of Númenór in SA 3319.
The Rohirrim were humans that lived in the country of Rohan. Physically, the Rohirrim were all described to be tall, blonde, and mostly blue eyed humans. They were fiercely independent and conservative, remaining within their own language and culture. Their horses were very important assets in their everyday lives with their entire culture being based around the breeding and trading of them. Their people lived mostly in villages on the plains of Rohan where their horses grazed. There were very few cities in Rohan.