Functions that are linear combinations of x.
$ g(x) = w^t x + w_0 $ Where $ w $ is the weight vector and $ w_0 $ is the bias as threshold. In the two category case $ g(x) $ defines the decisions surface where: $ w_1 $, when $ g(x) > 0 $ or $ w_2 $, when $ g(x) < 0 $
