A website has two primary ways of telling website search engines about what pages exist on the site.
This is a file that lists all the pages on your website, sometimes with a priority which emphasises the hierarchy of that site. It enables a search engine to quickly look at your website and consider indexing pages on it. Without it Google will get to the same result, but it just takes a lot longer (maybe weeks) as it will have to crawl all the links on your website to discover all those pages. Google only spends a small amount of time loking at a site before moving on, and the less frequently you add content the less often it will come back to check for new content.
A feed is another file that lists all the latest NEWS added to your website. It enables Google to look quickly at your site, spot new news and get it indexed, if appropriate, very quickly (important as news is generally of a timely nature). Without a feed Google might take a long time to discover your news and it might be out of date by the time it is indexed.
The interesting note I saw from Google was confirmation of best practice: have a sitemap.xml file AND a feed, AND sign up for Google webmaster tools and register both of those there. That will allow Google to get a quick understanding of your site structure AND for it to index your news in a timely fashion. PLUS, using webmaster tools you can submit pages by hand for consideration for the index (if they are new or have had a major update).
Google isn’t the only search engine on the block though, so you can bet the same advice is good for Bing webmaster tools too.
Make sure you have both these files hooked up to Google & Bing webmaster tools and know how to submit a page to Google webmaster tools when you make any big site updates or add new content.