The obvious difference is switches. And how they're packed/what installs them. Type 1: Typically for Core Component fixes, related directly to Windows. Type 2: Typically Component Updates, like IE, OE, WMP The following aren't really covered as there are only one or two for each Type 3: MDAC Updates. These look like Type 2, but aren't. The installer used is dahotfix.exe and requires different switches. Type 4: This one emerged with an XML update i believe. It's similar to Type 3, just a different installer file. Type 5 (as I call em): The newer version of Type 1. These are the ones that have the /quiet and /integrate type switches. These are just like Type 1, but an updated installer.