A filter is normally use say for example for compressed air-known as compressed air filter. They have a filter element(replaceable) of rated microns (say 5, 2 , .5). The lower the micron stated in element the finer the filtration and the expensive the cost of the filter. 1 micron is 28400th. part of an inch. what dust you see in the sun rays when you flick your shirt is around 300 micron. You cannot see more than less than 280 micron with the naked eye. This is lay-man way of understanding micron.

a strainer has a mesh brass/ss. Strainers are used prior steam traps, pumps, or any equipment you need to protect from damage by dirt/grit/scales etc.Strainers are installed as on-line and are in sizes 1/2" thru 12" or maybe more(I've used upto 10").

Filters (size rangeing 1/4" thru 2") won't work in place of strainers and strainers will be no good for filtering.

All strainers have differect mesh sizes to select from the manufacturer's chart.

Then there are filters for Ducts. These filters can go to 0.001 micron used in pharmaceutical plants in their air supply duct.Their sizes are the sizes of different sized ducts.

Filters for compressed air / water/ oil etc., are like glass bottles.

In Gas lines you come across both . first you put in a strainer and then a Gas filters. These gas filters are like fabricated vessels wth flanged top and they have a sponge like element like cut out of a roll in varoius thickness. These are put inside by removing the flanged top.

Strainers need periodical cleaning of screens and put back. Filters need replacement of elements after expiry of working life say 3-4 months. Duct filters have an indicator which fall down to read "replace filter" . Are you clear now? I'm tired.