There are three different types of wig constructions: custom made, hand-made ready to wear and machine-made.
Custom-made wigs are almost undetectable from natural hair. They are the usually the best viable option for hair loss patients, but they are expensive compared to ready made wigs and generally take from two / three weeks to a few months to complete.
Hand-made wigs look the most natural of the ready made choices because individual strands are knotted on to a net base rather than wefts of hair being sewn together. These wigs can be parted and styled with accessories because there is no chance the vents will be exposed. The hair also falls and moves more naturally.
Machine-made wigs are the least expensive and most widely available. Wefts of hair are sewn together in a straight line, cut and assembled into a wig. When you look inside a machine-made wig you can see the lines. Many look extremely realistic as long as they are not parted, pulled back or otherwise altered. Some people find these wigs more comfortable because the construction creates vents that allow air to circulate to the scalp.