No, unfortunately, this is not a posting on how to learn web design. In fact, it's not even an overview on how to learn the principles and practice of web design. Instead, it's a description on what does and doesn't constitute web design.
The most interesting thing that happens is a subtle mistake… people often confuse a Graphic Designer with a Web Designer.
First, the two roles overlap quite a bit. They both require an eye for design, layout, colors, and everything that makes up an eye-catching design. They must have the ability to conceptualize and express designs from verbal descriptions, wireframe mockups, and even napkin sketches while balancing the needs of the information to be represented with the limitations of the medium. A good designer – web or graphic – can make or break the effort. Few care about the boring desing… they usually just forget it.
Despite the similarities, they're not interchangable.
A Graphic Designer normally works towards developing an overall layout and the supporting graphics and colors. Quite often this is for a printed product but it can be for a variety of different medium… quite often physical but almost always static. In the past, their skill sets often overlapped with a Type Setter or they had one working in close proximity. You'll often find numerous Graphic Designers at ad agencies as they work with a customer incorporating or even developing graphical concepts and are integral to an organization's branding. For a microISV, your main interaction with a Graphic Designer will be mostly be in the branding area.
On the other hand, a Web Designer usually doesn't work with physical medium. They can also be involved with a customer in developing graphical concepts and can become integral to the branding process, but that's where it ends. They don't normally implement the designs, they usually hand it off to a web developer… but that raises another major difference. A Web Designer's concepts are imagined and implemented from the beginning to be dynamic. Interaction is expected and often the goal and one of the core principles of Web Design. Still being designers, they use some of the same graphics tools like Photoshop but the good ones will complement these skills with an understanding of HTML, CSS, and rarely even a bit of Javascript. The knowledgable ones will give you a div-based design with the accompanying css to handle all the details… the dated ones – or Graphic Designers disguised as Web Designers – will give you table with images and style information interspered.
Finally, in most organizations, you'll end up needing both at different times. When CaseySoftware had its extreme makeover last year, a graphic designer – Mike Rhode* – took the lead in eliciting feedback and information to guide me to a new logo and colors. It was an iterative process that involved a lot of "I like X better than Y". But as I've launched new sites, I don't often need those types of services. Generally, CaseySoftware customers already have a style guide defining logo, colors, and even fonts and it becomes the duty of a Web Designer to combine this document with usability to create an overall design.
A good Graphic Designer can make your logo distinctive, your advertisement memorable, or your box art appealing. A good Web Designer can make your site easy to navigate, straightforward to use, and something that people come back for. Each of these roles is vital but fundamentally different and cannot be interchanged easily.
* If you want some interesting reading, check out Mike's numerous descriptions of his process and even his design porfolio. He's well-established in the microISV community and even has a nice little pricing package for his services. He's top notch all the way.