Swimming goggles are meant to create watertight compartments around the eyes when under water. Salt water from the ocean, or chlorinated water from a pool cause irritation in the eyes which prevents easy swimming. It also most importantly allows swimmers to see under water.
Swimming goggles, however, cannot be used beyond a few feet under water due to the pressure that pushes them strongly against the eyes. For such purposes, most notably in scuba diving diving masks are used which serve very much the same purpose but tend to be much larger in size due to extra strength needed as well as the oxygen pipe attached.
While choosing a set of goggles it is important for an individual to keep abreast of a few things. The first thing is to determine their purpose. General swimmers would mostly choose a different variety than racing swimmers or divers. This is for the reason that goggles come in a range of costs. While they seem the same, many of them tend to let water in during high speed swimming. Also, many goggles tend to fog, due to perspiration and it is a good idea to keep an anti-fog spray around to take care of it.
Also, there are goggles with adjustable straps, while some come in fixed sizes where even the length of the nose piece cannot be altered. It is a very important consideration, and those with adjustable straps are preferred for the simple reason that they can be made to fit exactly.