In Raspbian Jessie, the default behaviour for all new installations is to boot straight to the desktop Graphical User Interface (GUI), rather than the Command Line Interface (CLI).
This was a decision taken because this is the expected behaviour for all modern computers; the default interface for a personal computer in 2015 is a desktop GUI, not just text on a screen. – Simon Long, User Interface Engineer
Because of this change, Raspbian Jessie now includes a graphical version of the Raspberry Pi Configuration tool which provides much the same functionality as raspi-config, just with a nicer interface! However, raspi-config is still available in Raspbian Jessie if you prefer use the command-line version.
This guide describes all the available options in detail, and is a companion to my Raspberry Pi Configuration tool (raspi-config) guide.