Before and After Properties of SharePoint Event handlers

Before events – Events that fire before an action occurs allow you to perform custom validation, checking, or processing of data that is about to be deleted, modified, or added to a list. These types of events are consistently suffixed with 'ing' to identify them as Before events. Note that the code reacting on these events is executed in a synchronous manner.

After events – Events that fire after a certain action occurs probably are familiar to most developers working with Windows SharePoint Services 2.0. These events are suffixed with 'ed' and the code handling them is executed asynchronously.