affect

usually a verb meaning "to influence" or "to impact"

• The hurricane will probably affect the whole town.
• When it doesn't rain for a longtime, it affects my allergies.
• This book really affected my thinking about that subject.

effect

usually a noun meaning "result" or "impact"

• The coach's attidue had an effect on the whole team.
• The fresh flowers had a beautiful effect on the room.
• A side effect of my medicine is drowsiness.