Some doubts about how to show posts in a custom theme?

get_template_part() is including a template file from the active theme directory.

For example:

get_template_part('content') will include the template content.php from the theme folder.

When having two parameters, it will search for a template named firstParameter-secondParameter.php. So:

get_template_part( 'content', get_post_format() );

will include the content-{$post_format}.php file.

get_post_format() returns one of this defined formats:

aside
chat
gallery
link
image
quote
status
video
audio 

or false. So the file included will be content.php or content-format.php