The service pack contains mostly bug fixes and small, incremental upgrades, as opposed to new features.
Microsoft timed the release to coincide with the formal debut of Office 365. You can read about the launch of Office 365 here, and find a plain explanation of just what Office 365 is and how it compares to Office 2010 here.
Office 2010 is the fastest selling version of Office ever, something that Microsoft wants to maintain. Whether this service pack will provide any sort of boost to sales remains an open question. TNW has long been bullish and positive on Office 2010. When it first came out, we had this to say on it:
If you’ve not yet used Office 2010, you should. I’ve been beta testing the productivity suite for a number of months, and while I have my gripes (who doesn’t?) it’s truly the best Office yet.
Now, users worldwide can get their hands on to Microsoft’s golden child, including Visio 2010 and Project 2010 as well.
Oh, a small warning: the service pack is between 300 and 400 megabytes, depending on which version you need, so you might want to get started downloading it.