A wetsuit top is quite similar to a rashguard except that it is made out of neoprene which can be anywhere from about 0.5 to 1.5 millimeters thick. While they are not going to keep you as warm as a full wetsuit, they are perfect for those times when you’re in water which is a little chilly or when the sun isn’t out or even on windy days. It can also be worn under a full wetsuit for added warmth and comfort while in colder water. A top can have either long or short sleeves.
Most people will wear a wetsuit top when the water is between about 72F and 77F (22-25C) so they will be able to stay comfortable even if they are spending long periods of time in the water. The stitching for a wetsuit top is usually the normal flatlock stitching which is not waterproof, but given the warmer climates this type of top is ideal for that is not a problem.
Most of wetsuit tops have a tightening system at the bottom to help keep them from slipping up over your belly while you’re surfing. You can also find a similar type of top which is called a wetsuit vest if you’re looking for something just a little different.
If you want to learn more about which wetsuit goes with which water temperature and what are the factors that influence the feeling of cold check this article.