Questions related to the publishing features - old school publish or publishing service. Questions about the publishing service specific should be tagged with the publishing-service tag.
When you edit or create new items in Sitecore, they must be published before they appear on your website. This lets you save unfinished items and decide exactly when the items are launched to the website.
When you work in Sitecore, the items you save are saved to the Master database. When you publish new items or updated items, the items are copied from the Master database to a publishing target, and from the publishing target, the website is launched. The default publishing target in Sitecore is the Web database.