Difference between dot and space in CSS

What is difference between
#tab1 a { text-decoration: underline; }
and
#tab.a { text-decoration: underline; }

#tab1 a { text-decoration: underline; }
<div id='tab1'><a>test</a><a>test1</a></div>
put all  underline for link under id tab1

#tab.a { text-decoration: underline; }
<a id='tab1'>test</a>
 Select link with id tab1 as underline

To remove underline
#tab.a { text-decoration: none; }