A tag selector selects element in the DOM tree.
/* Selects all level 1 headings */
h1
/* Selects all paragraphs */
p
/* Selects all strong elements */
strong
/* Selects all em elements */
em
/* Selects all divs */
div
/* Selects all mama's tag (who knows ?) */
mama
<!-- The p official HTML tag -->
<p>I will be blue</p>
<!-- <mama> is not a official HTML, XML tag but you can still use it -->
<mama>Mama will be pink</mama>
p /* select all p */
{ color:blue; } /* and color them in blue */
mama /* select all mama */
{ color:pink; } /* and color them in pink */