Simply put, there are three basic cases:
Starts with a '.' - refers to class
Starts with a '#' - refers to id
Starts with neither - refers to tag name
.myClass - this will select all items with class='myClass'
#myId - this will select the single item with id='myId'
element - this will select all items of tag element <element>
<style>
.firstItem { color: red; }
#secondItem { color: green; }
p { color: blue; }
</style>
<div class='firstItem'>This should be RED</div>
<div id='secondItem'>This should be GREEN</div>
<p>This should be BLUE</p>