Packages like Matlab or Scilab can solve those simultaneous equations for the intersection point if the slopes are different. This is another way of saying that the coefficient matrix has a non-zero determinant.
I might add that if you want to find two perpindicular lines you can still use the y = mx + b formula. However, perpindicular lines will be the negative square root of your other equation.
Lines which do not intersect are either parallel or skew.
Eric's answer is for lines drawn on graph paper - Lines in 2 dimensions. There are no skew lines in 2D. All pairs of non intersecting lines are pairs of parallel lines.
In 3D lines which do not intersect and are not parallel are called skew lines.
The simplest way to find out if two lines in 3 D do not intersect is to calculate the distance between them. If this = 0 they intersect, if it is +ve and nonzero they do not.