Less than 100gsm is called microns, over 220gsm is called board.
Copier paper is normally 80 gsm. It is cheap, bright and readily available in A sizes. It can be used in all printers.
Cartridge paper varies from 80-140 gsm, and accepts most media such as sketching and watercolour. However, it is more expensive than copier paper.
Tracing paper varies from 40-90 gsm and is used by designers. It has partial transparency (translucent), so designers can trace through previous design ideas to adapt and develop them. It is also cheap to buy.
Layout paper is normally 50 gsm and is cheap to buy. It has a smooth finish and is translucent, so you can partially see through it.