dispose spweb and spsite if you are not referring to the SPcontext
SPsite site = new Spsite("yourURL") < this should be disposed
spsite site = spcontext.current.site < this should not be disposed.
Event Receiver. IF you want to automatically do something based on SharePoint events. i.e on item creation you want to copy the newly created item into another list. you can do the action by adding codes into the ItemAdded Event.
Let me know if this helps
