This is often a catch-all term for painful joints, but actually means, "inflammation of a joint". "Arthrosis" is a more accurate but less well-known term for degenerative, often age-related change in joints.
Osteoarthritis: with age and overuse, certain joints can suffer a thinning of their cartilage and a lack of fluid lubrication. This can lead to stiffness and pain and a loss of movement in the joint.
Rheumatoid arthritis: a more serious inflammatory condition with swollen and painful joints, which can be associated with other health issues.
Gout: not as uncommon as people think, characterised by pain and redness, often in a peripheral joint such as a toe. It is caused by a deposit of uric acid crystals in the joint. It can be treated effectively by medication but underlying dietary and health issues should not be ignored.
Osteopathy can often help with arthritis and degenerative change by improving mobility of joints, strength in muscles, and encouraging activity.
More information can be found here.