Google makes a high number of changes each month (typically between 30-50) that impact the results you see. But the main ones that tend to worry companies these days are the Penguin and Panda updates. In the video below Google's Matt Cutts has said that Google expects to go deeper and have more impact with the approaching Penguin update. This really could be a cause for concern as the changes in recent months have had some of the largest impact to search results that I've seen in the past 10 years. So be ready for significant changes, and ready to react as quickly as possible should you be hit by these changes.