An "incremental publish" of a site only effects those pages that have updated since the last publishing run.
The "Publish Now" option allows you to publish time-sensitive changes to a content item or folder as needed. To use "Publish Now," right-click on any content item or folder, and click on the "Publish Now" menu item. If you do not see "Publish Now," it is because the item has never entered a publishable state (equivalent to the item being in the Public workflow state).
Publish Now on a folder will trigger a recursive, incremental publish of that folder. (Remember: incremental publishing only effects those items that have been changed since the last edition was published.)
Publish Now on a content item will publish the last public revision of the item.
We ask that you not abuse the Publish Now feature. Use the Preview menu item if you want to see what a page will look like before you publish it. Repeatedly editing the content item and triggering Publish Now, will only cause needless numbers of revisions of the content item to be stored.
"Publish Now" Publishing Priorities
"Publish Now" on a content item has a higher priority than a "Publish Now" on a folder. This means that a "Publish Now" on a folder will be paused while a content item publishes. Since a content item should take less time to publish than a folder, this prevents single content items from taking a long time (sometimes days) to publish due to the publishing of a folder.
Think of publishing in the CMS as you would with a newspaper. The public does not read the news until the pages are published. The CMS has two types of publishing runs.