Confusion over the power wires of USB (GND and +5v)
Unfortunately, when we talk about current flow it is opposite of the direction that the electrons move. It can cause a lot of confusion.

Here is an example circuit with a battery and a lamp. We usually use the following terminology:




You will almost never find anyone in the industry talking about electron flow. It is always just "current", flowing from the positive ("+") terminal to the negative ("-") terminal.
Because of this terminology, we say that the positive terminal is the "source" of the current.
Voltages are measured between two points. Imagine that the battery is 1.5V. If you connect your voltmeter with it's red probe on "positive" and the black probe on "negative", it should read 1.5V. If you reverse the probes, you will see "-1.5V".
In an electrical schematic, the battery's negative terminal is generally referred to as "ground" or "return". All other voltages in the circuit are measured relative to this node. It is sometimes referred to as "zero volts", or "0V", to make it more intuitive.