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 less than 1m long as adults, the largest enormous pliosaurs up to 16m long, comparable in size to sperm whales (Physeter). They were possibly the biggest predators of all time, though remains of these giants are rare and fragmentary.
In broad terms plesiosaurs come in two distinctshapes, the long-necked, small-headed plesiosaurs and the short-necked, large-headed pliosaurs. In the past it was considered that these were different families, but research since the 1980s has shown that both morphologies have arrisen more or less independenly on different lineages, meaning that some lineages of "pliosaur" are more closely related to plesiosaur lineages than to each other.
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 probable 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, ammonites and mollusks. Many specimens bear the bite marks of 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, robust teeth which were used to attack their smaller relatives.
We know that they gave birth to live young in water though the disovery of a single specimen with a large embryo in its body cavity, and the occasional finds of related but earlier species with embryos. From this it has been inferred that they looked after their young in social groups as such behaviour is typical of existing animals which give birth to small numbers of large infants.