Google's Udi Manber, VP of engineering, writes a very informative piece on the Official Google Blog about search quality at Google and the steps they're taking to make sure that Google's search quality is optimal. He explains that it has taken the engineering teams over 1000 man hours to develop without any stopping, and that's because the goal is to "improve the user experience." In this regard, there are a number of elements to search quality that are discussed.
Ranking is one of the more important elements about search quality, and Manber explains the difficulty of ensuring proper rank, particularly because of language ambiguity. The ranking algorithm utilizes PageRank and other elements (but Manber suggests that he won't give too much information away; this information is Google's "crown jewels" and providing too much would mean that the system could be gamed).
There's also a team that evaluates the effectiveness of Google, especially in different languages. Another team works on features and the user interface. Yet another team fights web spam. In other words, there are a lot of different pieces of the Google pie.
Matt Cutts takes this article to another level and illustrates the team environment. (I guess a cloud is better than pie.)
For someone who has an interest in the variety of teams working together to improve upon the Google experience, the post by Manber is informative. I suggest you read it. :)