CSS3 is used to control the style and layout of Web pages, making it easier to customize your pages in the future
CSS - a formal language for describing the format and appearance of a document written using markup languages. Mainly used as a means of defining the design or look of web pages dictated by the markup languages, HTML and XHTML.
In other words:
CSS - a style language that defines the layout of web pages based off of HTML documents. For example, CSS assigns fonts, colors, margins, lines, height, width, background images, positions and many other things. CSS3 is the third edition of the markup language that allows you to implement a variety of visual effects without the use of pictures.
What is it used for?
CSS3 can significantly reduce the size of your site, by avoiding the use of images in the design document. It also standardizes the resource by setting a fixed look for each class of elements, which greatly simplifies the development and further advancement of the resource.