A Strainer removes larger suspended particles from liquids. It is used primarily to protect downstream equipment like pumps from damage. Strainers are for larger particles. Filters are for smaller particles.
Filters and strainers for industrial use are used to remove particulates from liquids or gases. The size of the particles that strainers and filters remove is the crucial difference. Filters remove particles that the naked eye cannot see. Strainers remove larger particles from liquids and gases.