Empiricism is a theory of knowledge that emphasizes the role of the five senses in obtaining knowledge. That is, empiricists focus on knowledge gained by experience. Sometimes, this is called a posteriori knowledge. We are born as a blank tablet (tabula rasa) on which information is imprinted by sense data. Typically, empiricists do not give any credence to a priori knowledge (knowledge that we are born with or knowledge that exists prior to experience). One problem empiricists face is the role of the mind in receiving this sense data. If the mind is the blank tablet model, then it is a passive function. However, constructive empiricism, one of the different manifestations of this philosophy, holds that the mind can construct knowledge based on, and in addition to, the sense data gained by experience. Famous empiricists include John Locke, George Berkeley, and John Stuart Mill.
Rationalism is a group of views that emphasizes the role of reason, intuition and introspection rather than sensory experience. Rationalism began with the ancient Greeks and the extremism of rationalism was immediately confronted with problems such as language acquisition. A person might have the inherent capability for language but learning a language requires sensory experience. Therefore, rationalists had to admit the role of experience in confluence with mental reason. Descartes' famous “I think, therefore I am” is a rationalist statement but the words in the phrase were obtained from sensory experience. Famous rationalists include Rene Descartes, Immanuel Kant and Baruch Spinoza.
For the “rational” rationalist, the knowledge we obtain through our reason can include concepts obtained from sensory experience but knowledge can also be a priori. There is a biological or genetic ability prior to experience; so we are not born as a blank tablet. We are born with innate structures of knowing, one example being Chomsky's claim of our innate grammar.
Since Descartes, rationalists have had to come up with nuances to the theory in order to address how the mind and external world interact. The same could be said for the more recent philosophy of empiricism.