K&N Engineering (http://www.knfilters.com) are manufacturer
s of high performance reusable air filter
Typically, the filter element is made of a purified cotton substance laid out over a wire grid, and coated with a special mineral oil designed to trap particles as they pass through in the air stream.
These filters achieve higher performance by having a larger grid spacing than a typical paper or foam filter, and the particles that get stuck in the filter element tend to further enhance the filtration properties.
Eventually the filter will become too dirty to provide you any benefit over a stock filter. The filter manual suggests that every 20,000 miles (or 3 months under harsh conditions, ie: small, particulate dust. Like the farms near Interstate 5 in central California), that the filter be inspected, and washed as necessary, up to 25 times. At which point the filter should be replaced.
K&N also produces complete intake replacement kits, oil filters, and a host of other performance related equipment.