CSS selectors are used to "find" (or select) HTML elements
The three most common CSS selectors are the id Selector, class Selector, and element Selector.
id Selector
The following CSS selector will select all elements with an id "first-name"
#first-name
class Selector
The following CSS selector will select all elements with a class "credit-card-form"
.credit-card-form
element Selector
The following CSS selector will select all paragraph <p> elements
p
You can combine selectors as well
div.credit-card-form
Read more about CSS selectors on the web.