The difference between "#" and "." in CSS Files

I found this information on stackoverflow.com: basically if you have a style that you need to apply to more than one element, use the class. If it's something like a header that will be used only once, you can use the div id.

code in the webpage:

This will be red.

this will be blue.

code in the CSS file
#myRedText
{
color: red;
}

.blueText
{
color: blue;
}