Wetsuit jacket is similar to wetsuit top. The main difference is that wetsuit jacket usually has a front zipper that lets you completely open the jacket while you are putting it on or off or if you are feeling hot. Some wetsuit tops are also called wetsuit jackets even thoguh they have no zipper but that is kind of misleading. This mostly happens with surfing wetsuit tops which almost never have a front zipper. Why? While lying on your belly on your surfboard you would be lying directly on the zipper which would not be very comfortable. And because of the wax, the zipper would soon become dirty and hard to zip and unzip.

As you can see in the picture below there is a womans version of surfing wetsuit jacket that has only a short zipper below the neck. That kind of zipper won't bother you while surfing. An ordinary wetsuit jacket as we know it is in the picture to the right.

The other difference is that wetsuti jacket tends to be made out of thicker neoprene than wetsuit top and is therefor warmer. Zipper helps you get into the wetsuit jacket even when it is thick. Pulling a very thick jacket without a zipper over your head wouldn't work. Normal wetsuit jacket thickness is 2 millimeters while wetsuit top is usually from 0.5 - 1.5 (max) millimeters thick.

Wetsuit jacket is mostly used by divers. You can own a long john wetsuit and a wetsuit jacket and you can use them alone or combine them into a two piece wetsuit. This way you can cover quite a big range of water temperatures. 

The normal temperature range for a wetsuit jacket is somewhere around 72F (22C) but if you combine it with a long john wetsuit you can go much lower.