This help topic is also available in the form of a video tutorial which can be found here.
The Product Import Screen is used to both add new stock and to update existing stock details (quantities, cost prices etc.). There are two types of Import, Basic and Advanced. The Basic Import is fine for most basic tasks though you get more control and fields with the Advanced Import.
Basic Import Stock File - Learn more about it and how to use it
Advanced Import Stock File - Learn more about it and how to use it
eBay Import Products Screen - Learn more about it and how to use it
Common reasons for rejection - Learn about the most common reasons imports fail
Other files of Interest
Saving .CSV files using OpenOffice - Learn how to do your Imports with OpenOffice