A delta update updates the previous version of Mac OS X only. A combo update updates any version of Mac OS X since the last major reference release, such as Mac OS X v10.5 or 10.6.
For example, the Mac OS X v10.6.7 delta update can be installed on Mac OS X v10.6.6 only. The Mac OS X v10.6.7 combo update can be installed on Mac OS X v10.6, 10.6.1, 10.6.2, 10.6.3, 10.6.4, 10.6.5, or 10.6.6.
How can I install the correct update?
The easiest way to update your Mac is to use Software Update. It will offer you the best update package your computer.
If you are manually downloading an update from Apple Software Downloads, you can choose between the delta and combo updates: the delta update is a smaller download, but the larger combo update can be used on more systems.
See this article as well: Mac OS X: Software update sizes can vary.
Your system software will be the same whether you use the delta or combo updater, with one exception: If you are creating an image for NetInstall or NetRestore on several different Mac models, you must use the latest combo update when preparing the image, as described in this article.