What were some similarities and differences between Descartes and Spinoza?

Descartes and Spinoza were both rationalist philosophers. This means that they believed that the ultimate truths of philosophy could be established by reason. Rationalism is often contrasted with empiricism, which holds instead that sense experience is the source of truth. To rationalists such as Descartes and Spinoza, however, the senses often deceive us; they can never therefore provide us with the absolute certainty necessary for knowledge of the world around us. For example, that stick that appears bent in the water really is not actually bent, and the moon on the horizon that seems such a short distance away is actually hundreds of thousands of miles away.
Spinoza greatly admired Descartes and used him as a point of departure in his own work. As with the French philosopher, Spinoza constructed an elegant rational structure which purportedly dealt with the most important questions in early modern philosophy. A central element in both Spinoza's and Descartes's respective systems is the relationship between mind and body. Descartes attempted to solve the mind-body problem. In other words, how is it possible for something spiritual—the mind—to interact with a physical entity—the body? For example, we know that mental states are often followed by physical acts. However, if the mind and the body are ontologically distinct substances, then how does this occur?
Descartes attempted to solve the conundrum by seeking to demonstrate that the mind was necessary for the body's survival. As far as interaction between mind and body was concerned, Descartes posited the rather fanciful theory that it took place in the pineal gland, a small gland in the center of the brain.
For Spinoza, there is no mind-body problem. For Spinoza, the mind and the body are not two separate substances, as Descartes believes, but just finite modes or modifications of a single substance, which Spinoza calls God or Nature. Whereas Descartes separates mind and body, Spinoza unites them. Mind is simply substance as conceived under the attribute of thought, whereas body is substance conceived under the attribute of extension. As there is no real separation between mind and body, the problem of their interaction simply does not arise.
As a devout Catholic, Descartes maintained a fairly orthodox position in regards to the question of God. Belief in God was a matter of faith; questions regarding his existence were a matter for theology, not philosophy. Spinoza, on the other hand, was, as a later thinker described him, a "God-intoxicated man." God was an essential part of Spinoza's entire structure of thought. His conception of God was highly unorthodox and earned him the condemnation and opprobrium of many believers. These were not just Christian believers; these were also members of the Jewish community of Amsterdam, into which Spinoza had been born.
Spinoza was what is called a pantheist, meaning that he believed that God and the universe were one and the same. To orthodox believers of religion, this was nothing short of rank heresy; indeed, Spinoza was formally excommunicated by the synagogue of Amsterdam largely on account of his heterodox opinions. In Spinoza's day, most people conceived of God as existing outside of his creation, separate and distinct. By bringing God and his creation together, Spinoza was openly challenging the prevailing theistic assumptions.
On philosophical grounds, Spinoza regarded God or Nature as the one substance, something that did not depend on anything else for its existence. Only God fits this description. Descartes, however, treated mind and body as separate substances. To him, substances were things that had certain attributes. For instance, the mind has the attribute of thought, whereas the body has the attribute of extension, in other words, it occupies space.