On a basic level, the 5v of the USB port is regulated by the power supply of the external device, be it a computer USB port or a USB type charging module. When you are using the power jack on the Arduino, you can provide a range of voltage, from 7v (recommended minimum) to as much as 20v. This input power passes through a voltage regulator to provide the regulated 5v necessary to operate the Arduino.
Voltage is not "used up" by the Arduino, but you may have little reserve power for additional devices which are tied into the 5v bus.
Consider that voltage can be viewed in a manner similar to water pressure, while current can be compared to the volume of water flowing. If you have a 1000 mA power supply and the Arduino uses 500 mA, you have still available 500mA at 5v for other uses.
You can experience voltage drop in a circuit, depending on the way components are connected. That's more advanced than necessary for this discussion.
This is a very basic overview and details will vary from one model Arduino to another.
Also note that one should not apply 5v to other pins marked 5v as it bypasses the regulator and can damage the board or other devices attached.