CSS

Cascading style sheets are used to add the cosmetics or 'style' of a website. The code is kept independent to the HTML that makes the site work. CSS can be used to execute the same information in regards to the same type of HTML element. For example, if you want to always have the same border around any images you place on your site, instead of writing the same style code into the HTML, you can write it once using CSS and then refer the relevent HTML to it.
Using templates in Blogger, you can change many things; fonts, colours, column size, how many columns. What you cannot change using templates is the colour and style of links and how they will execute when clicked on eg. open in same/new window. You also can't insert your own background image, or tweak your columns, fonts, fontsize etc to your own needs or desires.
The term 'accessibility' in relation to web design refers to how many people have access to a website or page, in particular, people with a disability. The use of CSS combats the problem of the visually impaired as it separates the style from the basic information needed to make the page run. This allows the use of a screen reader to decode the information. Also, people without a fast internet server can usually load the basic HTML version of a site.
The problem I am finding though, in regards to coding HTML, CSS and to an extreme extent, javascript is that they each use a different language with different rules. You need to have a grasp on the language that each uses before you can have an insight to how your site is going to work. But I guess that's why we've got Dreamweaver.