There are two kinds of simple selectors. One is a universal selector or * and the other one is a type selector.
The universal selector replaces any single element in the document tree; if it used with another selector, i.e. attribute, then it is as valid as omitted.
A type selector refers to an instance of the element type in a document; for example an element <p> can be referred via a type selector written simply as p in which case every paragraph in that document will have same properties.
CSS simple selectors: