afternoon VS evening

Afternoon = the part of the day from 12.01 until 6 p.m.
Evening = The period of time at the end of the day, usually from about 6 p.m. to bedtime

Examples:

1) We have classes until 5 o'clock in the afternoon. (not in the evening)
2) It was 7 o'clock in the evening. (not at night)

*Night = the time between one day and the next when it is dark, when people usually sleep.