Lettering is really just drawing letters. Each stoke of every letter is carefully and deliberately drawn. How the letters all fit together and how things look as a whole are given lots of thought, too. And when you get done, you get something that's just fun to look at. Lettering is about aesthetics.
Writing is pretty much done without thinking much about the letters. Some of the letters may not even be legible, while others may be very poorly formed. Not much thought is given to how the letters fit together or how the words look on the page. When you get done, you just hope others will be able to read it. Writing is about communication.
Just some random pineapple facts above in some different lettering styles.