There's debate to this day within English scholar types as to when semi-colons should be used. Usually compound statements are the most used, although some atest that you can use them to start lists rather than a standard colon. Others want to phase it out completely. Personally I imagine it to work somewhere between a full stop and an apostrophe; when you're saying something related to the sentance, but need a suitable stop-gap to show your intentions of it standing as a singular statement.