Below are listed the top 5 extinctions on earth in order of most recent, to least recent.
Evidence suggests that the environment was taking a turn for the worse before the K-T extinction, it is widely believed that factor that pushed the situation over the edge was a collision by either a asteroid or comet. The second most common theory for the cause of the K-T event is massive volcanic eruptions on the Deccan Flats in India. Both scenarios would have caused havoc on the climate of the time, although arguments have been made that in the case of the asteroid impact, many animals above ground or water would have fried to death within hours of impact due to the intense heat.
Animals that thrived as a result of the lessoned competition include mammals and sharks. Both groups were previously limit in size and dominance, sharks by the large reptiles of the sea, and mammals by the dinosaurs of the land. Only after dinosaurs and the ocean faring reptiles such as Plesiosaurs, Mosasaurs, and Ichthyosaurs became extinct did large sharks and mammals appear (large sharks were also present pre-mesozoic era as well).
Before the Triassic-Jurassic extinction, mammal like creatures called therapsids ruled the land, while dinosaurs and their ancestors, archosaurs, were less numerous.
The Jurassic extinction event was thought to be caused by massive ongoing eruptions covering and area larger than Canada. This would have dramatically altered to climate, and many species could not adapt. Asteroid impact is a plausible reason for this extinction as well, but is not as commonly accepted as the eruption theory. This event has the least amount of scientists working on it, so many details are yet to be discovered.
The Permian-Triassic Extinction
250 Million years ago, the largest extinction on earth occurred, wiping our 95% (nearly all) life on earth.
It is thought that this extinction is also thanks to massive, sustained volcanism, located in the "Siberian Traps" which covered an area nearly as large as Australia. The result of this volcanism was massive global warming, which choked life out of the seas.
All life on earth is from the 1 in 10 species that survived this event. The earth took nearly 10 million years to fully recover from this event, and once again have a fully functioning ecosystem.
The Late Devonian Extinction(s)
The extinction at the end of the Devonian period occurred 360 million years ago and actually consisted of two consecutive extinction events. The two die-offs were between 100,000 and 300,000 years apart.
The Devonian period was a period of very warm seas, at an average of 93F (34C). During the two extinction events, the average temperature dropped dramatically to 78F (26C). This wrecked havoc the the primarily sea-based ecosystem of the time. Some theories for the drop in temperature once again include volcanism and asteroid impact, the ash resulting from these events would have blocked out the sun.
Plants, scorpions, spiders, and other similar creatures had all made it onto land before these extinctions. The first vertebrates also made it onto land before these extinctions, but was then wiped out. It wasn't for another 10 million years that vertebrates moved onto the land once again. The interesting thing is that the first amphibians were related to the coelacanth, while the amphibians that arrived 10 million years later (ichthyostegalians) are what we are descendant from. We were very close to having a completely different set of vertebrates ruling the land.
The Ordovician-Silurian Extinction(s)
This is the earliest and second largest of the 5 biggest extinctions on earth, occurring 444 million years ago. Like the Devonian extinctions, the Ordovician-Silurian extinctions also consisted of two consecutive events
The two die offs were caused by periods of massive glaciation, which caused sea levels to fall. The cause of the glaciation is theorized to be that the new influx of plants at the time pulled so much carbon dioxide from the air, that it resulted in a period of global cooling.
While it killed off more species than all but one other extinctions, the dominant groups prior to the Ordovician-Silurian extinction remained dominant after the event. This is in sharp contrast to the other extinctions which all caused a new lineage of species to become dominant.