Empiricism is a school of philosophy which holds that ultimate reality is derived from sense experience. As a philosophy it's closely allied with the methodology of natural science. The only kind of knowledge that matters for the empiricist is that which can be formally measured or verified. Everything that comes to our minds does so through the senses; our whole mental life is therefore derived from our sense experience. As such, we have no innate ideas; our daily experience simply writes itself upon the blank minds with which we are born. This process explains how we derive knowledge of the world outside us. Without sense experience there can be no knowledge.
Rationalism, on the other hand, does believe that there is indeed such a thing as a priori knowledge. That is to say we can have knowledge of certain and indubitable truths from the exercise of our reason, prior to sense experience. A paradigm example of this would be mathematics, where we don't need to rely on our senses to establish that 2+2=4. A priori knowledge is a higher form of knowledge, one that gives us access to a more substantial truth which transcends the everyday world.
For the rationalist, the senses often prove unreliable guides to the truth. For example, we may perceive a straight stick as bent when it is underwater—so, we need the faculty of reason to clarify matters. Reason is everything to rationalists, the ultimate standard of truth against which the world of sense experience is to be measured, and often found wanting.
Rationalist philosophers such as Spinoza constructed elegant, detailed, and elaborate systems of thought based on reason alone. Later empiricist thinkers such as Francis Bacon criticized such systems on the grounds that though they were internally coherent and intellectually rigorous they were entirely self-contained and didn't actually refer to anything "out there" in the real world, the world of objects studied by natural science.
Bacon uses a colorful metaphor to elaborate this point. Empiricists are like bees, in that they take items from the natural world and transform them into something different. Just as bees collect pollen and turn it into honey, empiricists take sense impressions derived from the world around us and turn them into ideas, concepts and hypotheses.
Rationalists, on the other hand, are like spiders. For just as spiders create beautiful, elaborate webs out of their own bodies, so rationalists construct intricate philosophical systems out of their reason. But as such systems are derived entirely from the operation of reason, they cannot tell us much about the empirical world of things and objects. Indeed, rationalistic systems of thought can only really tell us what's going on in the minds of the philosophers holding those ideas.