Not an ideology in the usual sense — conservatives have been monarchists, democrats, free-marketeers, protectionists, clericalists, and secularists — but a recurring disposition in political thought. Its canonical modern statement is Edmund Burke's reaction to the The French Revolution: societies are organic, the accumulated deposits of generations of custom and compromise; political wisdom is mostly tacit and local; and attempts to remake them from abstract first principles are likely to destroy more than they create.
From that root, several distinct conservative traditions branched. Anglo-American Burkean conservatism prizes constitutional continuity, localism, and prudence. Continental "throne-and-altar" conservatism (de Maistre, Bonald) was darker and more metaphysical, binding political order to religious truth. 20th-century Oakeshott gave the disposition its most elegant statement: to be conservative "is to prefer the familiar to the unknown, the tried to the untried, the actual to the possible." Late-20th-century Anglo-American "fusionism" welded Burkean traditionalism to free-market economics — an uneasy alliance whose strains are visible today.
What distinguishes all of them from Classical Liberalism is a deeper skepticism of individual reason as a political tool. Liberals begin with the choosing individual; conservatives begin with the inherited form of life that made choice possible in the first place.