Conquistador (meaning "Conqueror" in the Spanish language) is the term used to refer to the soldiers, explorers, and adventurers who brought much of the Americas under the control of Spain in the 15th through the 17th century.
After the discoveries of Columbus had gained Spain a foothold in America, expeditions were soon set out to conquer and evangelize this 'New World'.
The leaders of these expeditions are called conquistadores, a name that denotes that they felt connected with the reconquista, the Christian (re)conquest of the Iberian peninsula from the Muslim Moors (711-1492). They also evoked the name of Santiago Matamoros ("St. James the Moor-killer") before going into battle against the Indians, another echo of this connection with the reconquista.
Many of the conquistadors were poor nobles (hidalgos) looking forward to make fortune in the Indies since they couldn't in Europe.
The first Spanish conquest in the Americas was the island of Hispaniola. From there Juan Ponce de León conquered Puerto Rico and Diego Velasquez took Cuba. The first settlement on the mainland was Darién in Panama, settled by Vasco Nuñez de Balboa in 1512.
The most successful conquistador was Hernán Cortés, who in 1520-1521, with Native American allies, overran the mighty Aztec empire, thus making Mexico (then called New Spain) a part of the Spanish empire. Of comparable importance was the conquest of the Inca empire by Francisco Pizarro.
After this, rumours of golden cities (Cibola in North America, El Dorado in South America), caused several more expeditions to be sent out, but many of those returned without having found their goal, or having found it, finding it much less valuable than was hoped.
Some Spaniards, singularly the priest Bartolome de Las Casas defended Native Americans against of the abuses of conquistadors. Most of the conquistadors acted cruel towards the inhabitants of the regions they visited or conquered, killing, enslaving and otherwise abusing them. In 1542, New Spanish colonial laws were made to protect Indians. In 1552, Bartolomé de las Casas published "Short Account of the Destruction of the West Indies" (Brevísima relación de la destrucción de las Indias), which was used by the other European colonial powers, rivals of Spain, in criticism of Spain's role.
List of conquistadors and Spanish explorers, with the period and place of conquest or exploration: