"Borax" and "boric acid" aren't interchangeable terms. Though they're forms of the same substance, they have different uses. But both of them should be used with the same precautions you'd take with any chemical in the home.
Uses for Borax
Borax is a shortened term for sodium tetraborate decahydrate or sodium borate, which are natural compounds derived from the element boron. Appearing as a soft, crystalline powder, it's used as an addition to laundry detergent and soaps, as a mild pesticide -- particularly for ants and fleas -- and as an ingredient in making crafts.
Uses for Boric Acid
Boric acid, sometimes called hydrogen borate or boracic acid, is an acidic form of borax. It can occur naturally or be created in a lab in a refining process from borax with sulfuric or hydrochloric acid. It's a very weak acid with several medical uses, including use as an eye wash, as an anti-fungal for common fungal issues like yeast infections, and as a general disinfectant. Manufacturers also use boric acid as a preservative for wood, glass and other materials.
Boric acid is a more effective indoor pesticide than borax alone. In powder form, it works against fleas, cockroaches and ants by disrupting their nervous systems and damaging their exoskeletons. Different infestations may call for different applications of boric acid. For example, cockroaches generally cannot cross a "boric acid" line on hardwood or carpet.
To use boric acid for a flea infestation, sprinkle the powder liberally over floors or carpets on which fleas and flea eggs may be found. Leave it on overnight; then vacuum it up. The fleas should disappear.