What’s the difference between spell components and spell focus?

@ChrisPerkinsDnD what's the difference between spell components and spell focus?

— out of print DM (@OutOfPrintDM) October 28, 2016

A focus can substitute for a spell's material components, provided they aren't too costly. #WOTCstaff https://t.co/bgt7UWKcJr

— Christopher Perkins (@ChrisPerkinsDnD) October 28, 2016