What is a first-order system and what characterizes its response?

A first-order system has a transfer function with one pole: G(s) = K/(tau*s + 1), where K is gain and tau is time constant. Step response rises exponentially toward final value: y(t) = K(1 - e^(-t/tau)). The time constant tau determines response speed - after one tau, response reaches 63.2% of final value; after 5 tau, it is 99% complete. First-order systems have no overshoot. Examples include RC circuits and thermal systems.