Slim fit clothing refers to any garment that fits the wearer close to the body. Generous fit clothing is loose, whereas slim-fit styles are quite tight. Pants, shirts, and other garments in this style don't have extra fabric hanging or draping anywhere. People with lean body types especially appreciate the closer fit because it can give them a more tailored look.
Regular fit clothing is sized between the generous and slim fits. It tends to skim most body types comfortably; regular fit pants and shirts have a little extra fabric. For thinner people, slim fit clothing tends to look like regular fitted designs made for average body types. Without the availability of slim fitting clothing, many leaner people have to buy the smallest regular fit sizes and get them tailored to fit.