Events and sessions: cuộc + word, buổi + word (cuộc họp, buổi học)

Put cuộc before a verb and you get an event: something that happens, with people taking part. Họp (to meet) gives cuộc họp, a meeting; thi (to compete, to sit an exam) gives cuộc thi, a contest; gọi (to call) gives cuộc gọi, a phone call; hẹn (to arrange to meet) gives cuộc hẹn, an appointment or a date. Cuộc sống is life as you live it day to day; a person's whole life, from start to end, is cuộc đời.

Buổi is a part of the day. You know it from buổi sáng, buổi chiều and buổi tối. Before an activity, it's one session of it: buổi học (a class), buổi họp (a meeting), buổi tiệc (a party). It also counts sessions: Tuần này em học ba buổi. I've got three classes this week.

A few words take either. Cuộc họp sees the meeting as an event, buổi họp as a slot in your day; both are fine. A call or a contest is always cuộc, a class always buổi.

Sự also turns verbs into nouns, but for ideas rather than events; it has its own page.