Used at the end of a sentence to turn it into a yes-or-no question or to express doubt or a rhetorical question.

(colloquial) Used as an equivalent to 什么 shén​me in phrases like 干吗 gàn​má

(bound form) Used in transliterations, such as for morphine 吗啡 mǎ​fēi

Used at the end of a phrase to create a pause, often to signal that the speaker is considering what to say next or to highlight the preceding topic.