What is the difference between arthralgia and arthritis?

These terms are related but are not the same: ‘arthralgia’ is joint pain; ‘arthritis’ is joint inflammation. 
Many conditions cause both arthralgia and arthritis. Rheumatoid arthritis is a good example. 
However, arthritis doesn’t always cause joint pain. For example, an x-ray can show evidence of arthritis (such as osteoarthritis) despite the absence of pain.
Conversely, arthralgia is not always due to arthritis. Many common causes of arthralgia are not due to arthritis. Examples include injury, bursitis, tendinitis, and fibromyalgia.