A sticky is a thread that has a common asked question with very informative knowledge and answers. Also a lot of specific write ups on various subjects that are in depth and loaded with valuable information can make it to the sticky status. Sticky threads are created to help stop the creation of repetitive threads.
Stickey === is a post it on the outside of the filling cabinet The rest is inside the filling cabinet
The "computerese" of the term is that "sticky" is used when a post is "stuck" at the top of the category home page. It won't progressively move down, and more posts are added, like a "non-sticky" post will. Sticky posts are announcements, or informative posts deemed by the admin crew to be something we want to keep in plain view of all the users.