When the equation of a line is in Slope-Intercept form: y = m x + b, then m is the slope of the line and b is the y-intercept.
Explore what happens when you change the slope or the y-intercept by dragging with the mouse the colored dots on the sliders which represent the slopes(m-line 1, and n-line 2) or the y-intercepts(b,c--2nd line) of the lines below:

Exercises: 1. Make the first line with a positive slope and the 2nd line with a negative slope.
2. Make the two lines parallel. What can you say about their slopes?
3. Make the two lines perpendicular. Hint: The slopes have to be negative reciprocals.
For example, if line1 has a slope m, then the slope of line 2 should be -1/m.