Space probes are small packages of instruments that are launched from the Earth to explore planets. Probes have landed small instrument capsules on Mars and Venus. They take photographs or test the atmosphere of a planet. Some probes use the gravity of other planets to extend their voyages.
They pass close by a planet, using its gravity to swing around it and be hurled off towards another planet. Using this very technique the Voyager 2 space probe was able to visit Jupiter, Saturn, Uranus and Neptune. Probes do not have their own rocket power apart from tiny thrusters for steering.