Soviet Heavy said:
The guard arc is Pratchett writing about minor characters (in this case the city guards) and having an entire story be about the characters that normally just show up to be killed by the heroes.
The Witch arc deals with a coven of witches in a small rural kingdom in the mountains. It comments a lot on local politics and every day life for rural folk.
The Rincewind arc is a continuation of the first two books. It deals a lot with Rincewind going to exotic locals in the universe and getting into trouble, running away, and getting into more trouble. This might be a good spot to jump into the series and get used to his writing style.
The Death arc is about Death as a fully realized character and his wild adventures wanting to understand people better. Mort also deals with Death's adopted daughter and an apprentice he hires.
The Ancient Civilization novels aren't connected. Pyramids is a spin off on ancient Egypt, and Small Gods is a religious critique set in a place similar to ancient Greece.
The Industrial Revolution arc (as a side note I don't really like that title, they're more just stand alone novels than anything else)
-Moving pictures is about early Hollywood and movies in general
-The Truth is about newspapers and the press
-Monstrous Regiment is about Women in the military and society at large
-Going Postal and Making Money are the adventures of con artist Lupwig Von Moist and him being appointed to run the city's postal service and banks.
Read whatever sounds interesting, there really isn't a bad place to start. Personally I started half way through the Death arc with Hogfather.