From the start, you could embed data or import / export data between these programs. But the look of the documents was not easy to make consistent until Office 2007. The way the Office 2007 team has managed to make for a consistent look to documents is to tack on Themes. That's not the hard part - they also had to renovate some long standing differences in Templates -PowerPoint being the most odd. They also have to work in with the concept of Word Styles, new Table and Multi-Level List Styles. They enhanced Excel Lists and called the new feature, Tables.
At the end of the day, we need to grasp the interplay between the new Theme architecture and whatever Styles (as interpreted by each Office program) and Templates. Here's a help page that gives a very brief skim.
- Using Background Styles instead of Master Slide editing for Powerpoint
- Apply, Customize and Save a Document Theme in Word or Excel
BTW, seems Ed Bott and Woody Leonhardt have written a concise, insightful book going by the plebian name of Using Office 2007 - it doesn't go into verbal diarrhoea but it misses details as well.