what is a positive ground? what is the difference between + and - ground?

Answer 1

It is just a reference point to all other potentials (voltages) in the circuit.

In an automobile, the negative terminal of the battery is tied to the chassis/body of the auto. That makes it a 'negative' ground. Many components on the automobile rely on having the negative current return through the chassis metal.

If the positive terminal of the battery were to be tied to the chassis, then it would be a positive ground.

The telephone company uses 48 volt DC to run most of its land-line equipment. They tie the positive DC terminals of the batteries and power supplies to earth-ground (a "positive" ground). This cuts down on galvanic corrosion of metals other than copper (like steel wire shields, etc.).
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Answer 2

Ground is ground. It has no polarity. It may be tied to positive or negative terminals on a battery, but if it is tied to earth-ground, it is a 0 volt reference (normally).