Arthritis of the knee comes in two major forms: osteoarthritis and rheumatoid arthritis. Osteoarthritis is the most common, and it is a degenerative disease that affects the cartilage. Rheumatoid arthritis, which also affects the cartilage, is characterized by swollen, inflamed joints. Below is a table outlining the differences between osteoarthritis and rheumatoid arthritis. It is important to note, however, that overlap between the two does exist.