Pre-filters are usually supplied with twin-filtration systems and are designed to remove sediment and other suspended materials prior to the carbon filter. This prevents the more expensive carbon filter from blocking allowing it to remove more difficult contaminants. The pre-filter is designed to protect the carbon filter.
Sediment filters are available in a number of different pore or micron sizes ranging from 0.3 to 50+ microns. They can also be absolute or nominal micron ratings.