Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g. fonts, colors, spacing) to Web documents. CSS has a simple syntax, and uses a number of English keywords to specify the names of various style properties. This means that a css file sets the rules for the style your website will have and to set different fonts and colors, etc for your entire website. One you have these sheet done, if you want to make a style change you don’t need to go page by page making those changes, you can just change it inside the css sheet and it will automatically change everywhere.

Web Standards could be defined as an “ISO” that rules in websites making to have websites around the World Wide Web. When a web site or web page is described as complying with web standards, it usually means that the site or page has valid or nearly valid HTML, CSS and JavaScript. The HTML should also meet accessibility and semantic guidelines. Web Standards, make it easier to identify potential errors, lead to faster web development and building a better ground for maintenance. Now, these “standards” are NOT rules, they are recommendations by the W3c, it’s up to the developer to follow them or not.

Sources:
CSS
W3 CSS
Page Source, about CSS
Cascading Stly Sheets

Web Standards
What are web standards
About Web Standards
The Web Standards Project