This picture shows the two main sights of Avignon - the Palace of the Popes and the St. Benezet Bridge. Aside from these two sights, Avignon is mostly just a good jumping off place from which to explore the rest of Provence. It is a large city that is one of the main stops of the TGV train from Paris.
The Pont St. Benezet was the only bridge across the Rhone River in the Middle ages. It was mostly destroyed by a huge ice flood in the year 1668, so only a small portion of the bridge still stands. It only has 4 arches now, but it once had 22, so you can imagine how much larger it once was. From the end of the bridge you get a nice view of the town.
Between the bridge and the Palace of the Popes is a nice park called Parc des Rochers des Doms. This neat little park offers a beautiful view of the river and the famous bridge.
The Palace of the Popes (Palais des Papes) is the largest surviving Gothic palace in Europe. It was the center of the Catholic religion for roughly a 70 year span in the 1300's and 1400's. It is quite barren now with few furnishings to see, but the architecture is amazing.