ARISTOTLE
Aristotle (384-322 BC) was a major philosopher in classical Greece. He did work in all sort of subjects, from logic to metaphysics, from biology to poetics/drama. Since your question also asks about Hobbes and Locke, most famous for their political work, let me identify Aristotle's views on politics. Aristotle believed there were three valid types of government, depending on the size and scope of what was to be governed or upon local situations. (He studied the constitutions of various governments as part of his work in writing his work, Politics.) As state with a sole ruler ruling rightly is a monarchy. If that form of state is abused, it becomes tyranny. A state with a number of members of the ruling class is an aristocracy-- rule by the excellent ones, noble men suited for governing. This is how Greek city-states were intended to operate. If aristocracy is corrupted by having a few rule but not of noble character or in a noble way, Aristotle referred to that as an oligarchy (rule by a few). A state in which all worthy men participate in governing Aristotle termed a polity, a constitutional government. He saw it as a corruption, though, to have a full democracy (rule by the people), which would become the sort of thing we call mob rule.
THOMAS HOBBES
Thomas Hobbes published a famous work called Leviathan in 1651. The title "Leviathan" comes from a biblical word for a great and mighty beast. Hobbes believed government is formed by people for the sake of their personal security and stability in society. In Hobbes view, once the people put a king (or other leader in power), then that leader needs to have supreme power (like a great and mighty beast). Hobbes' view of the natural state of human beings without a government held that people are too divided and too volatile as individuals -- everyone looking out for his own interests. So for security and stability, authority and the power of the law needs to be in the hands of a powerful ruler like a king or queen. And so people willingly enter a "social contract" in which they live under a government that provides stability and security for society.
JOHN LOCKE
Today, Locke is most famous for his political theory. John Locke published his Two Treatises on Civil Government in 1690, following the mostly peaceful transition of government power that was the Glorious Revolution in England. Locke believed people are born as blank slates--with no preexisting knowledge or moral leanings. Experience then guides them to the knowledge and the best form of life, which includes choosing to form governments to make life and society better. This was his application of the "social contract" theory.
