Flickr does not provide any such facility as of now. Flickr users are allowed to upload files only and not folders. Even if you try to drag and drop the entire folder into Flickr Photo upload window, it won't accept it. It forces you to select individual photo or video files
However, if you still wish to upload entire photo folder to Flickr, you can take help of third party Flickr photo uploaders. They allow you to bulk upload photos to Flickr. PicBackMan is one such third party Flickr uploader, where you just associate the photo folder(s) and click upload. All your photos will be uploaded in forms of photostreams.
The plus point of using a third party flickr uploader like PicBackMan is, that if the folder on your computer has sub-folders, PicBackMan will upload them as individual photostreams without causing you any extra effort.
So, as far as uploading folders to Flickr is concerned, you can do so using external Flickr uploaders. But, if you wish to upload folders through Flickr's web interface, it is not possible as yet.