PPRuNe Forums - Help with Terminology.

Sorry I'm getting over excited after joining and this is my second thread!! but i'm just doing some re-caps and study from the start of the year for college and my manual's glossary includes Code share, it gives a definition of "This is when two airlines share the same flight- there will be two different flight numbers however they will both share the same aircraft"

I'm well versed with my terminology but i don't understand this so much!
Is it a scheduled thing or does it happen spur of the moment?

Thank you!

(also, just watching news coverage on the air crash in the hudson river, new york. Impressive save of all 150 pax! i'm still in shock though!)