While post excerpts can be displayed mainly on Homepage, they also able to be displayed on Category pages, search results and archives. Considering the fact that it would make no sense to display full posts on those pages since the reader needs to be able to scan through a larger number of posts to decide what he wants to read. Depending on your WordPress theme, if it is not displaying post excerpts instead full post, there is a very simple way to display post excerpts on wordPress pages.
Open the archive.php, archives.php or search.php. Find the following line:
<?php while (have_posts()) : the_post(); ?>
After that you shall find:
<?php the_content(__('Read more »'));?>
Change it with:
This is one of the very basic coding on creating WordPress theme. Try it!