It's a dispatcher, like what filburt'll be doing for WDFv6. Basically all URIs are sent through it, and it handles determining where things go how.
What do you mean, tuts on altering design? What design do you want to alter?
It's also worth it to start familiarizing yourself with plugins that are currently available. There are *many*, and each one makes your life a little easier by implementing functionality you can use, typically in an incredibly easy way. Some to keep in mind include restful_authentication, the authenticate plugin, ujs4rails, attachment_fu, and bunches of others. Shameless plug: magic_fields is a plugin I've developed for DRYing up form helpers. Keep in mind that DRY is a driving principle behind Rails!