Holsters made from plastic polymers come in all manner of styles, quality, and prices. It’s become such a popular holster material because of the near-perfect properties for a holster, as well as being easy to work with compared to other common materials.
Leather is a great material for making holsters, but has its drawbacks. Although various alternative materials to leather have been used (with mixed success) Kydex is by far the most popular alternative to leather.
Of course, Kydex is probably the best known and most popular material being used, but not all polymer holsters are Kydex per se.
Two popular polymer holsters are the Blade-Tech and Comp-Tac. In this video, I attempt to show the major differences between these two, using their popular OWB/belt slide style holsters as examples.
Both have their strengths and weakness, and as a rule, you get what you pay for with these products. Neither of these have any active retention, so probably best for CCW, off duty, or competition.