In most themes the main posts page —and in many themes the category and monthly archive pages as well— normally display a number of full posts. If your posts are long, you may prefer to limit the total length of the pages, in order to avoid excessive scrolling and/or better showcase your latest posts at a glance. It’s also advisable if you post many images or videos, as they may take too long to load.
One way to limit the length of the main posts page is to set a very low number of posts per page (in Settings > Reading). Another way is to use the “more” tag (if you’re not familiar with it, see here; pay very close attention to the notes on formatting issues: a broken theme because of improper use of the more tag is one of the most commonly reported problems in the forum).
With the more tag, what you get in the main posts page is the actual beginning of a post. The so-called “excerpt”, on the other hand, is an independent, custom made, summary. You write its content in the Excerpt module of the post editor screen (if you don’t see the module, click “Screen Options” top right to activate it). The content can be some text, or an image, or both.
The Excerpt box is an html editor. So if you want anything other than a simple text summary, you must work your way using html – or temporarily create and format the content in the visual editor of the post, then switch to html, copy the result from there and paste it in the Excerpt box.
Unfortunately, in the majority of available themes this function is dead. It only works in the following themes:
|Main posts page and Category/Archive pages
Autofocus [if post includes image]
Update: Plus the themes that support custom excerpts
Main posts page only
Sunspot [two main columns layout]
|Category/Archive pages only
• Shortcodes don’t work in excerpts; so you cannot have objects such as a video, an audio player or a poll.
• Excerpts in Chateau, Fusion and the Morning After work only if you have selected the relevant option in Appearance > Theme Options.
• In Steira, the excerpt also shows up when you view the single complete post.
• Hemingway is unique in that its main posts page only displays the beginnings of the two most recent posts (hence no links, formatting, images etc. – see note 3 here). The excerpt function allows you to override that: if you want complete posts in the main page, with their links and everything, you copy their whole content from the html post editor and paste it in the text box of the Excerpt module (as long as the content doesn’t include shortcode objects).
• Bueno can display excerpts in the sidebar if you add the Bueno Featured Posts widget (see here).
• Twenty Eleven and Duster display excerpts in the “Featured” post of the “Showcase” template.
• Selecta displays excerpts (or featured images) in the optional row that showcases your latest posts.