A washer has multiple purposes depending on it’s’ shape, form and material. Generally speaking, a washer is used to:
protect the surface of fastened materials,
spread the load/ pressure at the hold points of fastened materials,
assist with tightening and loosening of a fastener,
control the spacing of connecting components
lock and secure nuts in place (shakeproof washers)
A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers usually have an outer diameter (OD) about twice the width of their inner diameter (ID).
Washers are usually metal or plastic. High quality bolted joints require hardened steel washers to prevent the loss of pre-load due to Brinelling (permanent indentation) after the torque is applied.
Washers are important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces and phosphor washers are used for high abrasion and corrosion resistance.
'Type' is not to be confused with 'form' (but often is). The British Standard for Metric Series Metal Washers (BS4320) written in 1968 coined the term 'form'. The forms go from A to D for Bright Metal and denote outside diameter and thickness. They can be summarised as -
Form A: Normal diameter, normal thickness (thick)
Form B: Normal diameter, light thickness
Form C: Large diameter (wide), normal thickness (thick)
Form D: Large diameter (wide), light thickness (thin)