A filter is a device that removes particles from a given liquid or gas. It includes a disposable medium for removing particles of specified micron sizes. All strainers are filters, but all filters are not strainers. A strainer is only one type of filter. Strainer uses a perforated plate or screen mesh to remove larger particles from a process stream. The major advantage of a Strainer is that it is reusable. The filter screen is only used once and must be changed when it clogs.
The main difference seems to be in diameter of the holes in the media screen also termed as mesh size in case of strainer. There is no hard and fast size division to define strainers from filters. Filters capable of removing particles as small as 1 μm are available
Another difference is resistance. Strainers normally present low resistance to liquid flow. The pressure drop across most strainers is relatively small compared with the pressure drop across thick media filters or membrane filters.
Another difference is the function. A strainer is used to protect other downstream equipment (e.g., pumps, instrumentation) from damage by rogue junk. A filter is employed to separate particles from the fluid.
