Self publishing means that you had a couple thousand saved up to make a print run. Getting published means someone saw the marketing potential of your work.
you normally get more exposure with a publishing company thus potentially selling more volumes.

i have published 3 books, and am doing great for being an indie author. All you have to do is have a marketing plan and the time and willingness to see it through. Traditional Publishers are starting to go out of business because of it. The world of publishing is changing.