Well,before we starting to go with theme development we need to understand the Magento Hierarchy and where our theme falls into place. There are hundreds of folders and thousands of files, so the listing of all items will be impossible, I will only be focusing on the ones we need to work with.
app/design/frontend/base/default/ app/design/frontend/default/default/ app/design/frontend/<package_name>/<theme_name>/ skin/frontend/base/default/ skin/frontend/default/default/ skin/frontend/<package_name>/<theme_name>/
Within a sub-folder of app and skin are where our packages and themes can be found, a typical install of Magento comes with two packages base and default.
Next we can learn about packages.