  Unlike Whitney's definition of the corank-nullity generating function
$T(G;x+1,y+1)$, Tutte's definition of his now eponymous polynomial $T(G;x,y)$
requires a total order on the edges of which the polynomial is a posteriori
independent. Tutte presented his definition in terms of internal and external
activities of maximal spanning forests. Although Tutte's original definition
may appear somewhat ad hoc upon first inspection, subsequent work by various
researchers has demonstrated that activity is a deep combinatorial concept. In
this survey, we provide an introduction to activities for graphs and matroids.
Our primary goal is to survey several notions of activity for graphs which
admit expansions of the Tutte polynomial. Additionally, we describe some
fundamental structural theorems, and outline connections to the topological
notion of shellability as well as several topics in algebraic combinatorics.
