I answered this very question for someone on Webdesigner Forum so will run through my reply here but in more detail.

Floating is the css method of telling an object to “stick” to the left or right side of the website. Floating is very basic but means it’s easier to keep an eye on what’s happening.

Positioning is more advanced and has many options. You can assign individual margins and even “sticky” an object.

The problem with positioning objects is that it can be highly confusing, as you can end up with objects placed all over the place.
I only use positioning for elements that need to be placed where floating won’t help e.g. overlapping another object.

What do you recommend? Floats or positioning? and why?

Centering a website is quite simple, in your html create a div called wrapper and add some text e.g.

Now in your css add this,

#wrapper {
width:960px;
margin:0 auto;
text-align:center;
}


The width can be any amount as long as it’s less than 1200px (the most commonly used screen size of users). I have added the IE 6 fix. IE6 doesn’t like the margin: auto command so we need to add the text-align:center; command to combat this.

External stylesheet
This is the recommended method by me and many other professionals. The main bulk of the css stylesheet is kept in an external document. The document is called in the head of the html document. The document can be called anything but must end in the .css prefix, e.g. styles.css, some-random-file.css.

To call the stylesheet, we add the following code in between the two head tags in the html document.

The biggest advantage of using this method is the css is easily editable. You can link the stylesheet to as many files as you need. You can change the whole design of a whole website from one file.
The css code is also clearer to understand and can also help a website load quicker. This is the recommended method as websites become bulkier and they rely more on CSS.

Internal
This is the method recommended by many tutorial websites, but not by professionals. The css coding is inside the head tags of the html document. If your website uses a lot of css for the design, then this can add quite a few lines of coding to the document.

To use this method, we add the css code inside a style command, like the javascript command.

This method is handy if the document uses it’s own styles e.g. a javascript image gallery. Less css code included using this method is better.

Inline
This method is used quite a lot, as it is when using html. The earlier versions of html used this method to change the basic configurations of the website including fonts and colors.

The only time I recommend using this method is for adding some styling to a script, that cannot utliise stylesheets. This method isn’t recommended as it looks very untidy when reading through the code.

To use this method, we add a style command to an object, such as a paragraph.

All of the above examples are used by websites, the bottom 2 are used more by tutorial websites for ease of use. I always recommend the top option as it makes your html code look neater and makes it easier to edit the css code.

CSS was designed primarily to seperate the markup code and the design. CSS provides more control and flexibility for the presentation of the markup. The main advantage of a seperate stylesheet is that multiple documents can share the same styling. This means a whole folder of documents (or known as a website) can change it’s whole appearance from one file.

The CSS specficifications are maintained by the W3C foundation. The W3C is a not for priofit organisation set up by the internet’s founder, Tim Berners-Lee. The W3C maintains the specifications for all the markup and stylesheet languages used in Websites. These include xml, html, xhtml and css.

The current CSS standard is CSS 2.1 but CSS 3 is imminent. CSS3 brings many new techniques to css that used to be left to images and javascript. CSS3 is only used on the most recent browsers, Mozilla Firefox 3.5, Opera 10, safari 3 (mostly mac based) and Google Chrome. Unfortunately all versions of Microsoft Internet Explorer don’t follow the Css3 standards, but hopefully a new version will.

CSS is a major part of website design and development so is a handy language to know.