Olympus Mons is a dormant volcano on Mars. As far as we've seen, this volcano has formed the largest mountain in the entire solar system, almost three times as tall as Mount Everest. Olympus Mons stands 25 km tall with an edifice 600 km wide. For a reference point, Olympus Mons covers an area about the same size as Arizona. The summit of the mountain features six nested calderas, forming a depression 60x80 km across and up to 3.2 km deep. The edge of the volcano drops off in a sheer cliff, a feature unique in shield volcanos on Mars, of which Olympus Mons is one. Because it is a shield volcano, Olympus Mons is fairly flat, the average slope being only 5 degrees.
Olympus Mons was able to get so large because Mars doesn't have plate tectonics. On earth, plate tectonics keeps land masses moving over volcanic hotspots creating new volcanos instead of building on the same one, like around the Hawaiian islands. On Mars, the lack of plate tectonics means that the same hotspot will keep building on the same volcano for as long as its active, producing volcanic mountains like Olympus Mons.
The Mars Express orbiter has found lava flows on the peak of Olympus Mons ranging from 115 million years old to around only 2 million years old. In geological terms, 2 million years is fairly recent, so Olympus Mons might still be volcanically active, but we haven't seen any activity for quite some time.