From Adam Fields’s blog post, "Unthrilled with the Office 12 UI":
Over many years of designing custom content management interfaces for lots of people to use, it became crystal clear that there’s a huge difference between a “tool” and a “task”. A tool is a function that lets the user do something, but a task is a function that lets the user accomplish something.
In my experience, most successful content management interfaces are primarily task-based. When the user sits down in front of the computer, the goal is to get something done, not just use some tools. Tasks are for most people (beginners and power users alike), but tools are for power users. If you know what you want to do, but it doesn’t fit nicely into the framework of getting something done, you need a tool. Tasks should be the default.