As the name suggests, a reactive IT support approach involves reacting to issues after they have happened. For example, you are experiencing backup issues and so call for assistance to rectify the issue. In the time it takes for the issue to be fixed, your business suffers from a disruption to business continuity.
In contrast, a proactive IT support approach stays one step ahead by consistently maintaining IT systems and having warnings in place for any potential issues about to arise. Prevention rather than cure, if you like. When regular maintenance and servicing is in place, the downtime that businesses face due to technological issues can be majorly reduced.
Reactive IT support is generally billed as Time and Materials. Where Proactive would normally be a fixed price per device/user per month.