What is the difference between include and include_once?

The reason I ask is I have downloaded a script to be used with phpbb.. at the beginning it calls some of the messageboard config files to get some of the database info and put it into variables. The script originally says "include_once" for these files, but it doesn't pull the information into the variables. If I change those to "include"s it DOES see the information and fills in the variables. Later in the script, the query to the database still doesn't work, even though it is seeing the variables.

I'm just wondering how these two different inclues work and maybe find out the bigger picture of why I can't get this thing to work