Musee d'Orsay is not to be missed, even if you "know nothing" about art.  The museum boasts the best collection of Impressionist paintings in the world.   In room after room, some of the most famous paintings in the world are side by side.  AMAZING!   See a tour of the museum here.
You will definitely recognize these paintings, enjoy seeing them in this spectacular high ceilinged, bright, former railway station, and will be awed.  All the famous (& notorious) artists are represented here: Jean-Francois Millet, Gustave Courbet, Edouard Manet, Edgar Degas, Pierre-Auguste Renoir, Claude Monet, CamillePissarro, Alfred Sisley, Vincent van Gogh, Paul Cezanne, PaulGauguin, Henri de Toulouse-Lautrec, Auguste Rodin & his muse, Camille Claudel; WOW!   This is a don't miss spot in Paris.
Impressionism painting started in the 1800s.   Rather than formal indoor portraits or religious scenes, the impressionists focused on the use of natural light, the use of bold color, and outdoor settings.  Controversial at  the time, most people will recognize impressionist paintings for their color and gaity: activities like picnics, and scenery bursting with color, and on people enjoying themselves outdoors.  Paintings at the Musee Orsay by artists such as Monet, Renoir, and Van Gogh are world famous.
The Orsay is covered by the Museum Pass; closed Mondays  and to be avoided on Tuesdays if at all possible as it is nearly inaccessibly crowded on Tuesdays (the Louvre is closed on Tuesdays, so everyone visits The Orsay on Tuesday ).
Very big hint re entry to the museum:   The lines to buy tickets to enter the Museum can rival a line to get free tickets for a Madonna concert.  Think a 2 hour line.  But wait, there is a better way.  You can pre-purchase your tickets via the internet, or in person.  If you already have your tickets, you do not stand in the long line (because the line is to buy entry tickets).  Instead, you enter the museum via a special entrance "C" ...with no line.   You purchase your tickets via web---no same day tickets--- and then pick them up in a special area or onsite you simply walk by all the people in line, go to the special window, and buy tickets for the next day or later that week.  The key is that you cannot buy same day tickets, but as this is a "must see" Paris, and no one wants to stand in line forever, consider this option. Or get the museum pass, and you also use the C entrance with no wait.
 You can see this museum's wonderful offerings comfortably in 2 to 3 hours; it is not overwhelming like the Louvre.  Photography is not permitted.  Hundreds of pieces of artwork, paintings & sculpture, show-cased in a converted railway station.