The listings grabber is the code that provides TV listings info for MythTV.
What makes it a bit complex is that MythTV is used worldwide - and there are lots of different ways to get listings data. Fortunately there are already solutions that we can use, such as XMLTV and Schedules Direct in North America.
XMLTV is the work of Ed Avis and others. He's created an XML format that copes with the complexities of TV listings info.
XMLTV provides a variety of 'screen scapers' (1 or 2 for each country) - programs that visit a website and try to interpret the text as tv listing info. This is used by mythfilldatabase which then converts the XMLTV format and passes it to MythTV.
Some places provide raw XML or other machine readable text (eg the North American and the UK Radio Times grabbers) - this is much quicker and more reliable.