What many of us usually understand about tendinitis is that it's a condition involving inflammation of a tendon resulting from overuse and repetitive trauma to the structure. Responses to the diagnosis are typically to rest, apply ice for the inflammation, and then pop an Advil to further get that anti-inflammatory affect.
However, what we now know about "tendinitis" may surprise you. Rather than being an inflammatory condition (as the "-itis" suggests), we're finding the condition to be a structural one with changes to the tendon fibers occurring at the cellular level. Often, a complete absence of inflammation is observed, resulting in the research shifting to the terms, "tendinosis" or "tendinopathy", instead.
We can simplify the explanation by visualizing normal tendons as having neat, aligned fibers that are all closely packed together and running in the same direction. Normally, we think of tendinitis as this structure remaining, but inflammation and swelling simply occurring in the area due to applied stress. We know this as incorrect, though.
Instead, tendinosis involves that neat alignment of the fibers as becoming disrupted. In healthy tendons, there is regular fiber-growth activity replacing old fibers with healthy new ones in the similar fashion. In unhealthy cases, new growth of fibers is occuring in overtime (which is the reason for thickening of affected tendons) as well as growing irregularly and nonuniform, affecting the structure's functional ability and resulting in pain.