Traditionally, a witcher uses two swords, a steel sword for use on monsters not sensitive to silver or shelled so heavily that only the hard steel can harm them.
The steel swords which witchers make for themselves (or contract swordsmiths to make for them, usually Mahakaman) are traditionally made from either meteoric or magnetite iron, and grant the weapon no superior, or magical qualities. Geralt, however, does like to encourage the popular misconception that he wields a supernatural weapon, saying that it brings better pay for the work he does.
In the novels it is mentioned by Geralt that the steel sword is often mistaken for being primarily intended for fighting hostile people. It is true that witchers use their steel swords when fighting against armed opponents, but this is only because a silver sword is a relatively delicate instrument. Striking metal armor or parrying an opponent's weapon will quickly ruin a silver sword, unlike a steel sword which can withstand such use with little to no damage.
See also
Within the games, there are a number of steel swords that Geralt can use: