So what is CSS? "CSS, or Cascading Styles Sheets, is a way to style HTML. Whereas the HTML is the content, the style sheet is the presentation of that document.
Styles don't smell or taste anything like HTML, they have a format of 'property: value' and most properties can be applied to most HTML tags."
taken from
HTML DOG In simple word CSS can turn a bare text box into a fancy cool and stylish thing.
There are somethings CSS can do and some that cannot.
CSS can change color size position opacity(transparency) shape display or none-display (visibility)of the objects and the background, the borders, etc