what are the differences and similarities between RSS feeds and
shopping feeds (that are sent to comparision shopping engines like
shopping.com, mysimon etc.) I am looking for specific 'purpose' type
descriptions
as well as detailed technical differences. A sample of a shopping feed
and RSS feed and how they differ would be perfect.

The problem is that there isn't a standardised method used by the
different companies. Some use simple methods like tab delimited others
xlm. Regardless they are not going to have the same parameters
necessarily.

Like an example of a Froogle feed would be:

product_url	name	description	image_url	category	price
http://www.someweb.com/catalogue.php?item=1	Shorts		Blue,
Cotton	http://www.someweb.com/pics/Shorts.jpg 	Clothes		£7.00
http://www.someweb.com/catalogue.php?item=2	Golf balls	6 High
flight	http://www.someweb.com/pics/golfballs.jpg	Sports
Equipment	£7.00

Where the large spaces depict single tabs and you terminate with a new
line. This is how you could do it with php:

$header = "product_url\tname\tdescription\timage_url\tcategory\tprice\n";

$rows = "http://www.someweb.com/catalogue.php?item=1\tShorts\tBlue,
Cotton\thttp://www.someweb.com/pics/Shorts.jpg\tClothes\t£5.00\n";

$rows .= "http://www.someweb.com/catalogue.php?item=2\tGolf balls\t6
High flight\thttp://www.someweb.com/pics/golfballs.jpg\tSports
Equipment\t£7.00\n";

$frooglefeed = $header.$rows;
@file_put_contents('/temp/feed.txt',$frooglefeed);

Then you upload that feed to your froogle ftp. Of course in reality
you would probably loop though the items in you database to get the
rows.

An example and explanation of rss can be found here:
http://en.wikipedia.org/wiki/RSS_(file_format)

So in conclusion there is no universal feed. However there are scripts
out there that will help you upload your feed to the different sites.
Or you can create your own in php, etc. No feed is that complicated it
wouldn't be in their interests to make it so.

Hope that helps

If you want help with implementing the specific feeds not a problem

Paul