Some things just can't be improved. For example, Popeye's Chicken & Biscuits has terrific red beans and rice, and the only way to make them better would be a home-delivery option so I could enjoy them without leaving the house. However, some things can be improved: Airport security screening, the contents of Top 40 radio stations, and the attitude of US cell phone companies all come to mind.
Software is another thing that can be, and frequently is, improved. Sometimes these improvements take the form of major version releases or service packs; other times, we get smaller increments of improvement, such as patches or security updates. For example, Exchange Server 2007 SP1 introduced some major features (e.g., support for public folders in OWA), plus a number of bug fixes. Service packs for major products such as Windows, Exchange, and Microsoft SQL Server typically come out on a 12- to 18-month cycle, although the wait is sometimes much longer. Microsoft is generally good about producing Quick Fix Engineering (QFE) updates, or hotfixes, that address specific serious problems in between service pack releases. When a new service pack is released, it typically includes all the hotfixes released since the previous service pack. However, finding, installing, and tracking hotfixes across more than a handful of servers is a tiresome and error-prone process.
Enter the rollup. You can think of rollup updates as mini–service packs or as large hotfixes. They rarely include new functionality. Instead, their purpose is to collect all of the hotfixes since the previous service pack and package them in a single integrated delivery that can be tested and deployed with less overhead than deploying individual updates. Because the rollups are cumulative, when you install the latest one you get all of the fixes in that rollup, plus fixes from all previous rollups. For example, if you install Update Rollup 6 for Exchange Server 2007, you're also getting all the fixes released in rollups 1 through 5. The Exchange team's strategy behind rollup releases is described in the Microsoft article "The product service strategy for Exchange Server 2007."
The Exchange team intends to release these rollups every six to eight weeks. Update Rollup 6 for Exchange Server 2007 was released last week, which means you should expect to see the next rollup sometime in late April or early May. Each rollup is released with a Microsoft knowledge base (KB) article that explains what fixes it incorporates, as well as any special considerations for that particular rollup.
The rollup approach strikes a balance between consolidation of updates into service packs and demands from administrators to get fixes for important issues sooner. This approach also lets the Exchange team's sustaining engineering effort be parallelized so that fixes can be added to service pack releases while they're still in development. However, this parallelization only goes so far: It's not practical to develop a rollup for a service pack while the service pack is being created, as Nino Bilic explains on the Exchange team blog.
For that reason, Update Rollup 6 is only for Exchange 2007 RTM machines—there's currently no rollup for servers running Exchange 2007 SP1. If you're running SP1, you'll have to wait a bit longer to get the newest fixes included in rollup 6. Of course, if you're still running Exchange 2007 RTM, you can get many of the fixes included in rollup 6, plus a bunch of new features, by deploying SP1; that's probably your best option at this point.