If you deliberately lie about someone in order to harm them, your statement is slanderous. If you know your brother was home all day, it's slanderous to claim you saw him stealing candy at the supermarket.
When you say something you know is untrue about another person, that's slander—and saying it is slanderous. It's slanderous to spread malicious rumors about a business because you're mad at its owner, or to tell a lie about your opponent in the student council election to boost your chances of winning. In some cases, making slanderous statements is considered a crime.