In short it is an image distribution service for independent stock photographers. You send them your images then tell them which agencies to submit them to. You need to supply your login details for this part obviously. They will then send your images to the agencies, from where you can add categories, releases etc before submitting. What this cuts out is the need to upload the same images from your computer to the servers of the respective agencies. I only need to upload the images from my computer to the servers of Lightburner and they then do the rest. As someone on a very limited wifi allowance (15GB a month - nowhere near enough for serious uploading to ten or so sites), this saves both time and my precious upload/download allowance.
As someone unfamiliar with FTP, I found it a bit of a challenge to set up, but once I had achieved it the prog was remarkably simple. I would recommend ticking the box to tell it to put the images into the upload area rather than the queue, particularly on Bigstock as otherwise you will be stuck with the sometimes bizzare categories they autogenerate for you. You do this via the 'preferences' tab on each channel.
The main drawback with Lightburner is that it only stores your images for three days. You get an email before they delete them so you won't get too caught out. Online storage is expensive and as I mentioned before, this prog is free, so it seems fair enough. Mostly this shouldn't be a problem, but if a new agency opens up sometime in the future you would need to upload directly to them as your images won't be available on Lightburner any more. For this reason I won't be using their metadata editor, which is a shame as it would be quite useful to ex-iStockers like me and is also quite nicely laid out and easy to use.
The site is hosted via Amazon's servers so should be fairly secure from hacking (if you are worried about giving them your login details).