What is Rate law, orders, rate constant?

A rate law expresses the rate as a function of the concentrations. A reaction has an order when its rate law takes the form

the exponents $p$, $q$, … are the partial orders with respect to A, B, …, their sum is the overall order, and $k$ is the rate constant, which depends on temperature only. Orders are found by experiment; they need not equal the stoichiometric coefficients, and they can be fractional or zero.