I think there's a mismatch of terms, a linear function is a line of a form y=mx+b where when you plot this on graph paper (aka Cartesian coordinates) it's a line. At least y=mx+b is a general linear polynomial, or in this case a binomial., where m is slope of the line (rise/run) and b is the y-axis intercept)
There are also linear differential equations, but if you are asking about those you already know about lines in two dimensional Euclidian space.
A good general mathematics website is
http://mathforum.org/ , ask for Dr. Math and say that Bob sent you.
Rap