Toothpaste: Sodium Fluoride

Fluoride is a charged particle that integrates with tooth enamel crystals to make the tooth less soluble to acid. 

Sodium Fluoride is a fluoride "salt" (similar to NaCl, aka: table salt), and is the most common formulation of fluoride in toothpaste.

The American Dental Association approves sodium fluoride-containing toothpastes and allows them to advertise as "cavity fighting", etc.

Other than keeping your teeth clean, exposing your teeth to fluoride is the most important cavity-reducing measure one can take.