What is the difference between a plasma membrane and cell wall?

Aren't plasma membranes found in animal cells while cell walls are in plants? I believe the plasma membrane is more permeable than the cell wall and the cell wall has a more rigid structure compared to a plasma membrane?

The plasma membrane (cell membrane) separates the interior of the cell from the outside environment. It is the border between the cytoplasm and the environment. Cell membrane is  selectively permeable and able to regulate what enters and exits from the cell. http://en.wikipedia.org/wiki/Cell_membrane#Function
The cell membrane is consisted of 2 layers of phospholipids. Other components are proteins, carbohydrates, cholesterol.
m_en.svg" target="_blank">http://en.wikipedia.org/wiki/File:Cell_membrane_detailed_diagra m_en.svg
(the red things are the phospholipids. They form a double layer. Proteins and other stuff are placed between them as shown at the picture.)

Cell walls
Cell wall is located outside of the cell membrane. As we said earlier, all cells have cell membranes (the cell wall doesn't replace the cell membrane). Not all cells have cell walls though.
Cell walls provide structural support, are usually rigid, and blocks big potentialy harmful molecules from entering the cell. It also prevents lysis that might be caused by osmosis if the environment is hypotonic http://www.google.gr/imgres?imgurl=http%3A%2F%2Flegacy.hopkinsville.kctcs.edu%2Finstructors%2FJason-Arnold%2FVLI%2FModule%2525202%2Fm2cellfunctionandenergetics%2Ff5-13_osmosis_in_animal_c.jpg&imgrefurl=http%3A%2F%2Flegacy.hopkinsville.kctcs.edu%2Finstructors%2FJason-Arnold%2FVLI%2FModule%25202%2Fm2cellfunctionandenergetics%2Fm2cellfunctionandenergetics6.html&h=831&w=800&tbnid=rupVRgxbWHWugM%3A&zoom=1&docid=tfdsmzfunGBz4M&ei=7U9RVL6eG6L5yQOVkIAw&tbm=isch&ved=0CB8QMygBMAE&iact=rc&uact=3&dur=2115&page=1&start=0&ndsp=16
Cell wall structure is different from cell membrane. It is not consisted of the double layer of phospholipids.
The most well known cell walls are these of the plant cells. The main components of these are carbohydrates like cellulose, hemicellulose and pectin. Proteins are also present.

Cell walls are not only present to plant cells. Other cells can have cell walls, like:
http://en.wikipedia.org/wiki/Cell_wall#Algal_cell_walls
http://en.wikipedia.org/wiki/Cell_wall#Fungal_cell_walls
http://en.wikipedia.org/wiki/Cell_wall#Cell_walls_of_water_and_slime_molds
http://en.wikipedia.org/wiki/Cell_wall#Prokaryotic_cell_walls
(The components of the cell walls are not the same at every cell. Visit these sites of wikipedia if you are interested about the components of each cell wall.)