Question

What is difference between ground and negative voltage?

Answer 1

The ground terminal in a circuit is just a reference for absolute potentials at different nodes. It really does not matter if you call the potential at _any_ node as ground. So, if you wish you can call the positive terminal of a voltage source as ground. All the circuit analysis will still hold good. Defining a ground terminal will in no way affect the physical properties of the circuit.

The negative (or positive) terminal of a voltage source is relevant in the following sense. When you connect the two terminals of the voltage source to two nodes of the circuit, then there is a change in the physical properties of the circuit - potential & current. You are adding potential between the two nodes. So the negative terminal along with the positive terminal tells us the stepping up of the potential in that mesh.