7805+General ground question

Hello, nice forums you got here!
I want to understand ground.
1.Is a typical wallwart's (-) lead 0V or is the difference between the (+) and (-) let you call it 0V for ease?
2.Say I plug in this wallwart into a 7805, if the wallwart's (-) lead is not 0V, does that mean that the ground in my project isn't 0V?

Thank you for my silly question.

0V, ground or earth is just a reference point from where all voltages are measured. In single supply circuits, it's normally the negative rail but some times it's the positive rail.

For example, take the humble astable multivibrator,

**broken link removed**

If you replaced Q1 and Q2 with PNP transistors and reversed the polarity of C1 and C2, you would need to replace the +V with -V which in reality will mean reversing the supply.

A wallwart's output is isolated from earth or mains ground, so it is basically floating. Thus you can call either output "ground" (0V) as desired. Typically the minus connection is used for ground and all other voltages are referenced to that. If you connect the circuit ground to earth ground, then all voltages will also be referenced to earth ground.