Sitecore makes use of a source and a target database. In order for a content to be visible on the live site, a publish is required. This will transfer the item from the Source Database (Master) to the Target Database (example: Web). For more information about the Publishing Process, you can read my previous post here
Sitecore Event Queues are very important when you have many Content Management (CM) and Content Delivery (CD) servers.
The use of Event Queue is to synchronize the different servers when a change has occurred in the Sitecore Back Office.
How Event Queues work for CM?