There are a number of apps that are compatible with Office documents. As Simon said, Apple makes iOS versions of their own iWork apps (Pages, Keynote, Numbers) that can be purchased separately. There are also 3rd party combined suites that cost less than the separate Apple apps. Ones often recommended include Documents 2 Go, Quickoffice and Office2 HD.
If you want access to the actual Microsoft Office applications and can accept needing a network connection whenever you want to use the apps, a server-based solution such as CloudOn might be an option for you.
You'll find a number of other threads on the subject if you want to look for additional information and opinions.