What exactly is an observer pattern ? All the theory is accessible on Wikipedia. Basically we have an object you want to monitor ( observe ) for any changes. Most of the time this object just fires out events and we want to listen to them. We can have more objects observing one or more other objects so basically this can be M:N relationship
He compares them to triggers in the database world - bits of code that are activated when something changes on an object. His example implements an interface (IObservable) to set/get an email address and validate it with a regular expression. When the setEmailAddress method is used and the validate method is called, the observer kicks into action and checks to ensure the value is correct.