Mars represents energy and self-assertion: how you act, defend what matters to you, and express desire. The sign it occupies describes a style of action — direct and impulsive, patient and determined, or any other nuance depending on the sign.
Mars is neither "good" nor "bad": it is the force that drives you to act. A chart always shows it in context, never in isolation.
Domicile: Aries and Scorpio. Exaltation: Capricorn. Detriment: Libra and Taurus. Fall: Cancer. Mars is classed on the nocturnal side in the Hellenistic classification by sect (see the Venus entry) — a placement that comes as a surprise, since Mars is often perceived as "active" rather than "nocturnal" in the intuitive sense of the word.
Mars finds its joy in the 6th house (daily work, health, concrete effort).