Often people ask me what I do at work, and I normally lie and tell them "web design". I lie because very few people know anything about websites beyond what they see, and explaining these differences tends to make for a yawnful first introduction. In my opinion there are 3 parts to most websites:
1. Visual design
2. Realising (or "describing") the design (HTML)
3. Backend
Chart: http://photos-926.ll.facebook.com/photos-ll-sf2p/v133/234/81/193100926/n193100926_33700305_2920.jpg
The first item, "visual design" ("actual design" in the chart), is probably what most people think of when they think about web design. This is the "arty" bit, usually done in photoshop, where you choose colours, put this bit here, that text over there, etcetera.
The second part is one that people typically don't realise exists, in part because most people's experiences with web design are limited to WYSIWYG (what you see is what you get) tools such as Dreamweaver or Microsoft Frontpage, where visual design and realising the design are the same thing. The problem here is that websites are not large pictures or Photoshop files (although they are sometimes large flash files [1]) , they're described using a language called HTML (as in website.com/index.html). HTML is a bit of a hack and managing to describe your website to look how you want, in all web browsers, is a bit of a dark art [2]. This is the area I specialise in most (and the one that involves the most swearing - see pie chart). So why not leave this to tools like Dreamweaver? Mostly because they do a godawful job of it [3].
The third part (not depicted in pie chart) is rarely thought about, but is quite simple. When you, for example, add someone as a friend on Facebook, Facebook's servers do a few things - check that person hasn't blocked you, then make a note somewhere that you're friends with them. This is the "backend" to the website, the bit you can't see. I can also do this bit quite well :)
As I'm sure you can tell, there is a clear division between "arty" and "computer sciency" here - the first part is arty, and the other parts are for the near-autistic geeks with skills in programming. A lot of people claim to be good at both parts 1 and 2 - this is very rarely true. I can do okayish web design but I know lots of people are far better than me at it, and I know people very good at aesthetics etc but I generally kick their butts at HTML. It's a fairly common divide in people for them to be either good at art or good at engineering/science/programming, and it still applies here. People who think they're good at both and aren't need to be corrected as soon as possible, because otherwise they move onto thinking they can do the third part as well - and then it's only a matter of time until they create MySpace.
So, now you know what I actually do for a living, sort of. If you can think of a slightly more catchy title for it, please let me know!
Notes:
[1] Sites as large images or flash files are extremely bad for accessibility. People with poor vision often turn up the size of fonts on their screens, and blind computer users use programs called "screenreaders" to read what's on screen out loud to them. Neither of these work with images or flash files, and while this may not concern you, companies are under legal obligations towards accessibility.
[2] There are lots of browsers and basically always have been (the popular ones at the moment being Internet Explorer, Firefox, Opera and Safari). HTML developed organically by different browser manufacturers adding bits to it as they saw fit, which meant that a) it's messy and b) things don't always do the same thing in one browser as another. These days we have standards and very long, boring documents defining exactly how HTML should be treated, and most browsers obey these quite well. Unfortunately Internet Explorer is not most browsers, and even more unfortunately it's the most popular browser. It displays some HTML completely wrong, and is probably the leading cause of heart failure in web developers.
[3] As well as this, there is a bit of an elite culture amongst people who can do the second part themselves, regarding our superiority at handling technology than the beret-wearing mac-using web designer arty types, which is taken away from us if someone makes an easy to use program that does it for them. But it's mostly because we do a better job than the automated programs ;) Additionally, if anyone tells you that web design is a dead-end job, or going down the pan, they're probably talking about either part 1 or part 2. If they don't know part 2 exists, they mean part 1. If they know part 2 exists, they probably think Dreamweaver made it obsolete. Obviously at least one part needs to remain, so I'm not worrying about job security yet - especially as no one has replaced part 3 with a computer program yet!