A web browser is configured to categorise book marked web pages. The browser runs a process that provides markers (CM1, 2, 3), which generate marker signals corresponding to individual categories for the content of the web pages. A user interface (21) is operable to allow the user to collocate a selection of the plurality of marker signals with network address data for the web page to be book marked, and also to allow the user to retrieve book marked web pages on the basis of the marker signals collocated to them.