It is not necessary that there must be a curvilinear relation among variables,it can be linear or non linear correlation.

Positive correlation implies that both the variables will move in the same direction, either both will rise or both will fall.

Because both the variables are moving in different directions.

Because a negative value of correlation indicates an inverse relation.

Because it is very easy and simple to calculate correlation between two variables.

Because here we have to go to calculate correlation twice or thrice, with different variables.

Because due to this constant change, the data form a straight line on graph paper.

"r" denotes Karl Pearson's coefficient of correlation.

Karl Pearson was an influential biometrition and statistician who has been credited with establishing the discipline of mathematical statistics.

As 1 shows perfect relation between two variables and the + sign shows positive relation