Surface Filters
Surface filters work by direct interception of particles larger than the pore size of the media. Dirt is trapped on the upstream side of the media with the holding capacity limited by the number of media pores. When new, media resistance of flow is small, but as the filter builds up with contaminants, the resistance to flow rapidly increases.
Depth Type Filters
Depth filters work by both direct interception of particles and absorption (molecular attraction of particles). These filters use several types of media to achieve the goal of holding particles. The fluid must take a longer path through the filter before exiting. Normally, these filters have large holding capacities and initially have a higher resistance to flow.