CSS, or Cascading Style Sheets is a method developed to allow for laying out web pages without the use of tables. With tables, the html code used starts to get really cumbersome, and confusing. Also, once developed, these pages can be confusing to edit later on.
With CSS, sections of a web page can be sectioned off and given certain characteristics, like width, height, background color, etc. Some of these sections can be directed to “float” to the left, or to the right. CSS greatly simplifies the code, and makes edits later on much easier to implement. CSS is much more readable than complicated table structures.
With CSS, you can put sections within sections, similar to way you can put tables within tables to create a layout. Yet, when this is done with CSS, the sheer volume of code is greatly reduced, and the remaining code is simpler and easier to read.
Besides losing all the table code (<tr>, <td>, etc.), you lose all of the descriptive code that is required when not using CSS. All that code is located in its own area, either within the head of the html document, are in an additional .css file, that is referenced inside the html page head.