hmm, "chaos theory" is such a vague term really. in essence it is non-linear dynamics, so it is arguably the study of any system whose equations are non-linear. there is more to it than that, obviously, but there is no reason that we cannot use the styudy of non-linear systems to model things like markets, indeed i believe they do.
if you're not familiar with linear and non-linear then let's have an example.
a pendulum is modeled by an equation
\ddot{\theta}=k\sin\theta
where \theta is the angle to the vertical of the "string" this is a nonlinear equation that we cannot solve so we linearize it and replace \sin\theta with \theta a linear equation we can solve. non-linear dynamics is essentially trying to study the harder equations without this approximating step,