They fall into two general categories, thermoplastics and thermoset plastics. Thermoplastics become soft and melt at elevated temperatures. They retain their shape and will not become soft or melt like thermoplastics. You must also be aware that all plastics possess a characteristic called 'creep'. The tendency and amount of creep can be 'ballparked' by knowing the flexural modulus of the material you are working with.
Flexural modulus is a measure of the material's softness or brittleness.
The higher the flex modulus the more brittle or rigid the material will be and the less creep it will normally have. Thus the flex modulus can be used as a general guide as to what type fastener is needed to make your threaded joint in plastic. Generally you can 'guesstimate' the flexural modulus with a sharp pocketknife. If you cut along a sharp edge of the material and get a nice curl the flexural modulus is around 200,000 or less and you can usually use a thread former. Also most plastic manufacturers do not have a good understanding of the relationship between threaded fasteners and plastic parts.