How to create ClickLink, FillOutForm, VisitWebPage activities

I'm trying to understand what each activity type is from the list of the activies defined in the Marketo web service WSDL.
Is there a document or article which describes how to create these activities ?

I'm particularly interested in the activites ClickLink, FillOutForm, VisitWebPage. Is there an article on how to perform these activity types.