Plesiosaurs were aquatic reptiles, and were an important part of the marine ecology from the end of the Triassic period 220 million years ago until the end of the Cretaceous 65 million years ago. Their remains have been found on every continent.
A typical plesiosaur had a long neck, a broad body, four large flippers and a relatively short tail. An apt description (of some forms at least) is of "a snake strung through a turtle". Plesiosaurs were one of the first kinds of extinct animal known to science, and were described as early as 1821. The smallest were about 2m long as adults, the largest were enormous pliosaurs up to 20m long, comparable in size or even bigger than sperm whales (Physeter). They were possibly the biggest predators of all time, though remains of these giants are rare and fragmentary, and much research remains to be done.
They are a large and varied group which have been broadly defined as two distinct types, the long-necked, small-headed plesiosaurs and the short-necked, large-headed pliosaurs.
Most of their remains have been found in horizons laid down in shallow coastal waters or estuaries. This does not mean that they all lived in such environments, merely that the chance of being fossilized if you die in such an environment is relatively high. It is probably that the smaller forms fed in productive coastal waters, and that the larger forms hunted them. During much of the period during which the plesiosaurs dominated the seas the major land masses of today's earth were closer together so that one major ocean far larger than the modern Pacific dominated the marine environment. We don't find fossils from the ocean floor, and there is no way of knowing what was happening out in the open ocean far away from land.
We can learn about what they ate from the fossils. Occasionally stomach contents are found with well-preserved specimens, showing that some at least fed on belemnites and ammonites. Other specimens show bite marks from larger predatory forms. There is a wide range in tooth shape, showing that they were adapted to feeding on different types of prey. Long, slender teeth may have been used to rake through sea-floor sediments in the way a swan dabbles on pond bottoms. Larger, more robust teeth seem to be adapted for feeding on armoured fish and cephalopods. Some of the bigger pliosaurs had enormous, dagger-like teeth which were used to attack their smaller relatives.
Nothing is known from the fossils of how they bred. Ichthyosaurs, which lived at the same time, gave birth to live young, and fossils of ichthyosaur mothers with newborn embryos are sometimes found. No such specimens of plesiosaurs have ever been discovered. Ichthyosaurs were unable to leave the water to lay eggs - they were as adapted to their way of life as are dolphins - but it is possible that the smaller plesiosaurs at least could crawl onto sandy beaches and lay their eggs like modern turtles. If this was the case, it is highly unlikely that we will ever find the evidence - conditions on a sandy beach are not good for potential fossilization.