This is an example of a Jon Renau open-capped wig. Not all open-capped wigs are built exactly this way, but they are all very similar. This is to give you a general idea of what to expect.
The biggest difference between an open-capped wig and one with a standard / traditional cap construction -- because they are very similar -- is that in a standard wig, the framework of the wig is constructed and the tracks of hair are sewn into it. This makes it heavy. Furthermore, the top of an open wig (like the one above) is wefted, just like the sides and back, and that allows the scalp to breathe whilst keeping weight to a minimum. In a standard wig, the entire top is covered in easily identifiable rose lace, which can enclose the scalp and trap heat. An open cap is a level up from a standard cap in terms of weight reduction and comfort.