What do publishing companies do?

If you have a book that has been professionally edited, graphic designed etc.. do you need to go through a publishing company to be able to sell the book or can you just start selling it on your own?What do publishing companies do?
you can try but would find it difficult to place it in shops - publishing companies have contracts with them making it much easier to place and they also promote the book



this article tells you much more about the process than I could fit into an answer here



http://en.wikipedia.org/wiki/Publishing

d3100

the pillars of the earth