Service Pack 1 combines a number of updates and improvements into a single download, so if you haven’t been keeping your computer up to date thus far, this one download is a good way to catch up. The download runs around 537MB for 32-bit users and 903MB for 64-bit users, and contains a number of bug fixes and security patches. Feature-wise, don’t expect to see a lot of changes — it looks like HDMI audio performance is the only fix most people will notice. For a full list of the hotfixes and security updates, you can download this Excel spreadsheet and hunt and peck for anything you may have hoped to see fixed.
To install it, just open up Windows Update and it should prompt you to download the service pack.
Windows 7 and Windows Server 2008 R2 Service Pack 1 [Microsoft Download Center]