When I first started modifying WordPress themes, I did that the wrong way, which involved editing existent themes with my styles, and how painful that was when one client did an auto-update for the theme a while later washing out all my work!
Then I took a step further by building my own themes based upon blank themes, those themes give you a fully functional WordPress template with no styling at all, and this went quite well for some time, although it was kind of doing a lot of work each time with the basic structure and formatting for all elements over and over again. I used Blank theme by Chris Coyier, then Starkers theme by Elliot Jay Stocks.
Recently I heard a lot about the concept of Child themes, and soon it became my favorite way of creating WordPress-based sites most of the time, and here is why…