工作 (gōngzuò) is the general word for 'to work' (verb) or 'a job' (noun) — it covers any kind of employment or task. 活 (huó) is more colloquial and often refers to physical or temporary work; 打工 (dǎgōng) specifically means working a part-time or odd job, especially for students. Use 工作 for formal or broad work situations.