Deism is difficult to specifically define, but in general, Deists believe that there is a God, but that we can only rely on reason to perceive any religious matters. Most Deists also believe that God only revealed himself through the creation of the universe, and has not revealed himself in any other way since.
Deism was particularly popular in the US in the 1700s and 1800s as a distinct school of religious thought that was a response to both atheism and traditional theistic religions. Some of the founding fathers were considered deists, and many scholars believe that at least several of the early US presidents were either Deists, or on the borderline of being Deists.
Note: The people quoted here are not necessarily deists.
Thomas Paine
(1737-1809) Influential English-American writer
Do we want to contemplate his power? We see it in the immensity of the creation. Do we want to contemplate his wisdom? We see it in the unchangeable order by which the incomprehensible whole is governed. Do we want to contemplate his munificence? We see it in the abundance with which he fills the earth… In fine, do we want to know what God is? Search not the book called the Scripture, which any human hand might make, but the Scripture called the Creation. (From Age of Reason)
Thomas Jefferson
(1743-1826) 3rd American president, scientist, writer, and lawyer
Question with boldness even the existence of God; because if there be one, he must more approve of the homage of reason than that of blindfolded fear.
(1817-1862) American writer, philosopher, and naturalist
There may be Gods, but they care not what men do.
Matthew Arnold
(1822-1888) English writer
All things seem to have what we call a law of their beings; whether we call this God or not is a matter of choice.