Cargo pants are loosely-cut pants

Trousers

Trousers are an item of clothing worn on the lower part of the body from the waist to the ankles, covering both legs separately...

 designed for tough, outdoor activities distinguished by multiple cargo pockets.
A cargo pocket is a form patch pocket with accordion folds for increased capacity closed with a flap secured by snap, button, or Velcro common on battledress

Battledress

Battledress, or fatigues in the general sense, is the type of uniform used as combat uniforms, as opposed to 'display' dress or formal uniform worn at parades and functions. It may be either monochrome or in a camouflage pattern...

 and hunting clothing.

Cargo pants are made of hard wearing fabric and ruggedly stitched. Increasingly they are made of quick-drying sythetic or cotton-synthetic blends, and often feature oversized belt loops to accommodate wide webbing belts.

The garments are characteristically designed to allow bending at the knee and hip and sewn with felled seams.

Cargo shorts

Shorts

Shorts are a bifurcated garment worn by both men and women over their pelvic area, circling the waist, and covering the upper part of the legs, sometimes extending down to or even below the knee, but not covering the entire length of the leg. They are called "shorts" because they are a shortened...

are cargo pants shortened at the knee. Some cargo pants are made with removable lower legs allowing conversion into shorts.