Posterior Tibial Tendinitis is not a common tendonitis, but typically occurs in runners. The main symptoms are either/or swelling and pain just above or below the medial malleolus. This is an overuse injury so the cause is when the load over time in the tendon exceeds what the tendon can take, usually when not enough time is given for the tendon to adapt to changes in load. It is typically associated with ‘overpronation’ if the forces of pronation are high.
This rant on video from Podiatry Soapbox a good overview of the condition and how it should be treated.
The management is not that difficult provided common sense and a logical sequence of events is followed, yet have been a lot of questions in forums and Q & A websites (eg here, here and here) with a mixture of the quality of advice being given online.
One of the biggest problems with posterior tibial tendonitis is runners is that in many places they confuse it with posterior tibial tendon dysfunction when they are two totally different beasts.