the_post_thumbnail(); // without parameter -> Thumbnail
the_post_thumbnail('thumbnail'); // Thumbnail (default 150px x 150px max)
the_post_thumbnail('medium'); // Medium resolution (default 300px x 300px max)
the_post_thumbnail('large'); // Large resolution (default 640px x 640px max)
the_post_thumbnail( array(100,100) ); // Other resolutions

http://codex.wordpress.org/Function_Reference/the_post_thumbnail