It started back in 1995 as a joke between friends while playing racquetball. They'd encourage each other by using pirate jargon.
They were soon discovered by a nationally syndicated columnist, who loved the idea and decided to feature them in a column. Since then "National Talk like a Pirate Day" has mushroomed into what it is today.
Here's some basic terminology,
-Avast-stop and give attention
-Aaaarrrrrr! - which can mean everything from "I'm happy" to I just sat on a safety pin.
If you've ever wanted to find out what your pirate name is, click here.