Most of this post only applies to you if (a) you have a Windows 7-based computer, and (b) you have the 64-bit operating system.
A "Service Pack" isn't a new product; it's a major release of a given software product that offers updates, bug fixes, and/or enhancements. While many such items may be delivered from time to time by way of the regular Windows Update function, a Service Pack, well, packs everything into a single installable file. You install the one file, and all the goodies that come with it are installed. You may/may not notice any differences in your software–be it Windows, or perhaps Microsoft Office–following the installation.
The first Service Pack for the Windows 7 64-bit operating system (a mammoth file, by the way) contains quite a few items, but the following two may be most recognizable to most people:
- The connection between Windows and HDMI audio devices has been made more robust, and a fix has been instituted that constantly maintains the connectivity.
- XPS documents (a proprietary Microsoft format for documents that is similar to PDF) were sometimes not printing properly when the documents were mixed-mode; meaning, that there were both portrait and landscape pages in the documents. This glitch has been addressed.
Other changes made were a little on the exotic side, at least for most users (things like the behavior of “Restore previous folders at logon” functionality and support for Advanced Vector Extensions AVX).
After the requisite reboot, I can't say my 64-bit version of Windows 7 Ultimate looked, 'felt', or ran differently. Still, it is highly recommended that you install most recommended updates seen in Windows Update, and especially Service Packs.

