Vico is famous for his concept of truth as an act, verum factum. His major work is the New Science (Scienza Nuova), published in 1725 and, as a completely rewritten work, in 1730. His major idea - that truth is an act, is discussed in this work as a principle of history - that it is made by humans, and so humans are able to fully understand it, just like mathematics, which is product of human imagination. Vico suggested study of tradition, mythology and language as methods for investigating history. He is believed to be the pioneer of ethnology. The historical life follows similar pattern - although not in details - in every nation, according to Vico. The first humans thought in mythical terms, universali fantastici or poetic characters. All nations begin by fantasia, the power of imagination and the age of gods which are needed to comprehend the world. After that, there comes a second age in which fantasia is used to form social institutions and heroes are used to inspire moral virtues. The third and final age is the age of rationality, in which humanity declines into barbarie della reflessione - barbarism of reflection. According to Vico, this is a cycle - gods, heroes and humans - which repeats itself within the world of nations, forming storia ideale eterna - ideal eternal history.
Vico was an isolated genius, who lived in near poverty and never met a thinker of equivalent magnitude. However, his views have influenced many philosophers in the ninghteenth century. Karl Marx studied Vico, and owes much to him. Also, Vico's notion verum factum anticipates pragmatism and pragmaticism of Peirce. He was able to predict historical development of Europe with appalling accuracy.
Further Reading: Isaiah Berlin ([1979] 1981) Against the Current (Oxofrd: OUP), in chapters of "Vico's Concept of Knowledge" and "Vico and the Ideal of the Enlightenment".