The Summer 07 release included a feature that protects your data from a common source of data corruption. In an on-demand world, a user may be updating data long after they initially retrieve it from the Force.com database. Update Collision Detection automatically checks to make sure the data being updated has not changed since it was initially retrieved. There are some cases where you wouldn’t want to do this, but the feature handles those properly too.
Read all about this feature in this article.