1. A computer-implemented method for managing an advertising campaign, comprising: registering a first event handler to receive event messages of a first event type from among a plurality of event types, wherein each of the plurality of event types is a distinct type of event resulting from one or more occurrences related to a web page that is associated with user interaction with the web page; registering a second event handler to receive event messages of a second event type from among the plurality of event types and not of the first event type from among the plurality of event types, wherein event messages of the first event type result from occurrences related to a first type of ecommerce activity on the web page and event messages of the second event type result from occurrences related to a second type of ecommerce activity on the web page; receiving input from a user of the web page via a communications network, wherein display content of the web page is generated by one or more display applications executing on one or more servers and is transmitted to the user via the communications network; determining, based on the received input, one or more events associated with web page activity of the user to be events defined by the advertising campaign; one or more server applications executing on the one or more servers generating a first event message of the first event type based on a first determined event of the determined events, including associating the first event message with the first event type and identifying the first event message as the first event type by including identifying information in the first event message; the one or more server applications executing on the one or more servers generating a second event message of the second event type based on a second determined event of the determined events, including associating the second event message with the second event type and identifying the second event message as the second event type by including identifying information in the second event message; forwarding at least the first event message to the first event handler; forwarding at least the second event message to the second event handler; the first event handler modifying operation of the one or more display applications, based on the forwarded first event message being of the first event type, to generate first modified display content comprising at least a first advertisement from the advertising campaign; the second event handler modifying operation of the one or more display applications, based on the forwarded second event message being of the second event type, to generate second modified display content comprising at least a second advertisement from the advertising campaign; and transforming the display content of the web page to comprise at least one of the first modified display content and the second modified display content to present at least one of the first advertisement and the second advertisement to the user via the communications network.