In reality it is a constant process - the change or updates the word we are most familiar with. There was a recent update by Goog;e that caused a lot of noise. But than search engines are constantly evolving and changes are evident.
The search engines are trying to place the most relevant and interesting results up on the result pages. Ths mean lot of quality inspection, lot of content parameters and consideration of factors which help in qualitative analysis of an interface. If your site is on the worng side of the guidelines you will suffer.
Unlike earlier on page factors are being greater weightage and hence site building is now a professional job. cheap web designing and neglected approach means you website will fail online. Some of the articles I have bookmarked here will help you understand my point better.