Understanding the syntax

I'm afraid I've hit a point with the theme code where the PHP tutorials and the Injader Help Wiki can't help me. 

First, where are functions defined?  I seem to remember they're all in a file somewhere, but I can't find the file nor directions to it in the Help Wiki.

Second, please explain the following from index.php:

1   if ($CMS->TH->GetContentCount() > 0) {
2     $strContentWrapperStart = <<<ContentWrapperStart
3 ContentWrapperStart;
4    print($strContentWrapperStart);

I understand that line 1 causes the subsequent code to be executed only if there is at least one article in the area.  In line 2, I know this is defining a variable, but what is <<<ContentWrapperStart ? I can't find anything about <<< (and it's hard to search; Google doesn't exactly recognize it as a keyword, and I don't know what to call it).  And in line 3, why is there just a label with a semicolon?  Is this a function?  Is it calling something from the database?

I'm not planning to modify this code, but understanding what the syntax is supposed to be (and whether I have inadvertently messed it up) will help me with troubleshooting my (frequent) PHP errors.