The Setup Assistant
What you see depends on whether you installed Mavericks onto an older OS X drive or an empty drive.
After Installing over Snow Leopard, Lion, or Mountain Lion
If you installed onto a drive that already had OS X stuff on it—accounts, files, settings, and so on—then after the Mac restarts, all you have to do is log in as usual.
You’re asked for your Apple (iCloud) name and password, or you can sign up for one now. You’re asked if you want to turn on Find My Mac (Find My Mac, Find My iPhone) and if you’d like to use the new iCloud Keychain feature (Organizing Bookmarks in the Editor).
It’s possible, at this point, that Mavericks will display a message warning you that it found some incompatible software on your Mac during the installation. You’ll discover that this outdated software has been summarily tossed into a folder called Incompatible Software in your startup disk’s main window.
You may also be asked if it’s OK for certain programs to access your calendar, reminders, or contacts—non-Apple programs you already own. Other programs may require that the Assistive Devices feature of OS X be turned on; you’re directed to the appropriate pane of System Preferences, where you’re supposed to turn on the appropriate checkboxes. All of these are security steps, new in Mavericks.