Well, on a glance.... I'm sure there are people with more elaborate explanations on this....

Discovery: is a vertical scan of your infrastructure trying to find all discoverable configuration items (servers, routes, printers, etc...). Discovery will bring limited relationship information.

Service Mapping: is a horizontal discovery. You start with a business service and (once configured) ServiceMapping will follow the services configuration to it's lowest config item (usually some SAN...). This gives you a complete and actual picture of your service configuration.

Event Management: collects events from an external monitoring solution and relates them to your CI's - with some additional logic to rate/qualify these events. Using Event Management and Service Mapping you can identify which service is affected by an event.