In both cases, if you have set anything in the Excerpt meta box on the post editor screen, that text will be used. Otherwise, the excerpt will be automatically trimmed.

The most commonly used method is the first one, because the user editing the post can choose between showing the whole content or just the excerpt, individually for each post. However, using the_excerpt() in your template file can also get a teaser from the first 55 words of the post content, without the need to place a more quicktag in the post.

To add a more quicktag in your post, put your cursor where you want to end the excerpted content of your post and click the more quicktag button. quicktags are the little buttons found above the editing window in your Administration > Post > Add New Post. They include bold, italic, links, and others, and the famous more.