Does someone you know always shrug in a particular way or say "You know what I mean?" at the end of every sentence? Well, that's a mannerism, a habitual gesture or way of speaking.
Generally, we don't use then term mannerism as a critical term for our friends' behavior, but when applied as a term of art criticism, it can be a negative thing. If a critic talks about a writer's mannerisms, for example, it's probably intended as an insult — suggesting they're writing in a lazy or clichéd way. However, Mannerism, with a capital "M," refers to a specific 16th-century Italian style of painting known for its artificial (as opposed to naturalistic) effects.