I'm a bit behind on my various blog posts here so for some people you may think this is old news, but for plenty more there are still lots of fab new features that have come into WordPress over the past 18 months, and as I haven't written about them yet I want to start now
Rather than writing out and repeating how to do everything, I've decided to just link to the tutorials and sites that I used to learn these features, and just add my own notes instead. However, of course I'm always happy to answer questions and help via the comments section.
This came in WordPress 2.9. Before this specific option, you could still create a featured image by attaching an image to a page or post via the media uploader and then using a bit of querying to pull it out and display it. Trouble was, it was a bit of a hassle and hard to explain to clients or less advanced users. Bring on the Featured Image. A way to allow posts and/or pages to have a single specified image that just required one line of code in your theme file to display.
I think the best post on this subject has to be from Mark Jaquith – Post Thumbnail Images (note, this was their name before it changed to 'Featured Image'). He covers everything you really need to know.
Featured images brings a whole host of new options. It's far simpler to create a photo blog with them for a start, and I can remember helping people out in the past where they used the categories and posts system to hold product details for a catalogue style site. Before it was a case of upload and attach the product photo (or several) to the post, and explain to the client to put the main one at the top using the order system, so that you could code in the theme to pull the first one out larger, and then perhaps show small thumbnails of the rest. Now you can simply tell them to use the featured image which can easily be displayed in product listings, and as a larger image on the main product page, with any additional images added via the media uploader and displayed as thumbnails.
This is just a simple example of how this can be used