Lines that supply various voltages for an electronic circuit are referred to as rails. Typically, each rail is designed to provide a specific voltage at currents ranging up to a maximum number of amperes (milliamps, et cetera).
Every modern electronic system will include a ground rail -- this is the "0 volts" point from which all other voltages in the system are based. A system will almost always include a +5 volt rail and/or a +3.3 volt rail, as these voltages are commonly used for logic -- although lower voltages such as 1.8v are also becoming common for newer logic gates. It may also have a +12 or +15 volt rail to drive speakers, motors, et cetera; and one or more negative rails (-5 or -12 volts are common), also often used for speakers when rail to rail operation is desired. In addition, there may be one or more rails specifically designed to handle high current loads, or rails for analog circuitry that should be shielded from digital noise (including a separate analog ground line).
Often you will see the main positive voltage rail for a system designated as "Vcc" or "Vdd". This has to do with the labeling of BJT and FET transistors: "Vcc" would connect to a BJT collector, and "Vdd" to a FET drain. Older datasheets may also use "Vee" and "Vss" to represent the negative rails, being connected to a BJT emitter and FET source, respectively. Nowadays, "Vcc" is still commonly used for the positive power rail and "GND" for the ground line, although you will sometimes see "Vdd"/"Vss" in datasheets for components such as operational amplifiers.