You will find it spelled nahual, nagual and nawal. There are many different definitions but you can think of nahuales as Mayan spirits or totems. Your nahual is assigned to you at birth and has a heavy influence on your character and misdemeanor much the same way the zodiac does. Each nahual is associated with an animal of importance to the Maya as well as associated traits, qualities and tendencies.
A person's nahual is determined by their date of birth based on the Tzolk'in calendar. Here you can find an accurate nahual calculator.