To be more elaborate:
Microsoft regularly issues updates to windows software (I believe once in a month). After a period of time, it issues a huge patch called "Service Pack" that incorporates all the updates that have been issued for the Windows software. This is useful for new installations. Even after the last Service Pack (for WinXP, we had SP2 a long while back), newer Windows updates are made available every month. These typically had to be downloaded one at a time and patched using Microsoft Support website...
The Autopatcher software takes Service Packs to the next level by providing all the updates up to the latest issues in a single huge patch. Also, in the Autopatcher software, you can choose which updates to omit when you find it unnecessary and I dont think you can do it with the Service Pack. Autopatchers are typically available either as full version (all updates included) or update version (only last 1 month's updates provided)
Arun