Product bulk import file format is basically a tab delimited text file. You can find the sample import file from
here.
The file with the column headers is pretty much self-explain. Make sure you need to have all the columns separated by tabs even if the columns are empty. If you have SKU defined, the bulk import process is going to check if there is existing product with the same SKU. If so, the existing product is updated. If not, a new product is created. So you could use bulk import process to bulk updated product stock level if you fill out the SKUs.
Here are some notices to certain columns.
1) Category
You could separated multiple category levels by using the special character ":". For example if you put Barrettes:Crystal, it would create Barrettes category if it doesn't exist and then create the sub category Crystal as a child of Barrettes category.
2) Image_Urls
You could separated multiple product image Urls by using comma. For example if you put http://domain.com/image1jpg,http://domain.com/image2.jpg, two images will be created for the product and first one will be the default product image.