Scott,
"+ve" and "-ve" are commonly used abbreviations for positive and negative in parts of the world which came under British influence.
Ali,
There are various reasons for adopting positive or negative grounding on DC systems, probably the most well-known one being the electrolytic action which can be set up, resulting in the erosion of conductive materials.
That's one reason why almost every telephone system in the world works with a DC supply which has the positive pole of the battery grounded.
When it comes to supplies for feeding certain electronic equipment, there is also the question of whether the primary devices used in the equipment require a supply line positive or negative with respect to earth.
From an historical viewpoint, valve/vacuum tubes operate with a positive supply line, and thus have a negative-ground supply, whereas much early transistor equipment used PNP transistors which required a negative line and thus a supply with its positive side grounded.
