A CSS Selector selects which elements a set of CSS rules applies to. The universal selector (*) applies to EVERYTHING. An element name selector uses the name of the element with NO preceding symbols (for example, h1 would refer to first-level headers).
For example, say you wanted all first-level headers to be centered. You would have the following:
The "h1" in red is your selector.
Class selectors start with a period ".", and selects elements through the "class" attribute. These refer to groups of elements (grouped by way of having the same class)
ID selectors start with an octothorpe (also known as a hashtag, "#"), and selects an element through the "id" attribute, the value of which is suppose to be unique within the webpage.
I wrote an entire chapter on them in my tutorial.