Algebra/Slope
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Slope is the measure of how much a line moves up or down related to how much it moves left to right.
In this image, the slope of the blue line is a / 1 or 
.
Parallel lines are those that have the same slope and do not touch. Examples include latitude lines.
[edit] Slope
Algebra/Slope
Slope is the change in the vertical distance of a line on a coordinate plane over the change in horizontal difference. In other words, it is the “rise” over the “run” or the steepness of a line. Slope is usually represented by the symbol m like in the equation y = mx + b, m the coefficient of x represents the slope of the line.
Slope is computed by measuring the change in vertical distance divided by the change in horizontal difference, i.e.:
The Greek uppercase letter Δ represents change, in this case change in the y-coordinates divided by the change in x-coordinates.
Positive Slope/ Negative Slope
If a line goes up from left to right, then the slope has to be positive. For example, a slope of ¾ would have a “rise” of 3, or go up 3; and a “run” of 4, or go right 4. Both numbers in the slope are either negative or positive in order to have a positive slope.
If a line goes down from left to right, then the slope has to be negative. For example, a slope of -3/4 would have a “rise” of -3, or go down 3; and a “run” of 4, or go right 4. Only one number in the slope can be negative for a line to have a negative slope.
Other Types of Slope
There are two special circumstances, no slope and slope of zero. A horizontal line has a slope of 0 and a vertical line has an undefined slope.
Horizontal lines have the form: 
; where a is a constant, i.e. 
Vertical lines have the form: 
; where as is a constant, i.e.
