stylesheet enables web authors to attach styles(like fonts,spacing,colors) to html documents.style sheet not only enhance your site but also reduce so much work for the web developers.for example you have 100 pages in your website.all have <h1> tag with red color.if you go on specifying the color to all 100 pages makes you bore. thats why if you have style sheet and specify color in the stylesheet. all the pages get affected.
How to Use Styles
When a browser reads a style sheet, it will format the document according to it. There are three ways of inserting a style sheet:
External Style Sheet
An external style sheet is ideal when the style is applied to many pages. With an external style sheet, you can change the look of an entire Web site by changing one file. Each page must link to the style sheet using the tag. The tag goes inside the head section.
Internal Style Sheet
An internal style sheet should be used when a single document has a unique style. You define internal styles in the head section with the <style> tag.
Inline Styles
An inline style should be used when a unique style is to be applied to a single occurrence of an element.