HTML is the language used to make a simple web page in it's most basic form.
Javascript and PHP are scripting codes to make a page do something, almost anything really.
XML is another language that's pretty ugly as is. An XML file could contain yourname, myname, and someone else's name. It would have design symbols in it like the "greater than" and "less than characters". So here's this ugly file that's pretty useless as is but but but the fascinating thing about XML is XML has been around "forever". Since it has been around so long, you can pass the info from a webpage to a program to a spreadsheet. It opens up all this sharing of information which is great. Back to the ugly file, so what it has the names I want to use, and it serves a purpose then.
RSS- Really simple syndication is a fancy form of XML(Atom is too) that sites use to send out "feeds" of news.
AJAX is a cool way to update a webpage without someone leaving a webpage. For example, suppose you submit a form at a site. Instead of waiting for a page stating that you just did submit the form, we've passed that(obviously a good point since we all knew we filled out the form), now we just apply the values in the form right there on the very same page.
CGI is an old script type language (for the sake of this article) like Javascript and PHP. It's used to make pages do things. It's older than old but, people are updating things from time to time and it still has it's usefulness.
CSS is a way to style a webpage.
XSL is a way to style an XML document.
MySQL is a real common type of database. A database might be where you'd save a spreadsheet and on a web page you'd fetch just part of the spreadsheet to display the info. The database stores the data.
Classes or API's or UIs or Libraries are packaged pre-made scripts. They're wonderful shortcuts to make coding simpler. You've probably seen your share of menus on webpages that look similar. Imagine that everyone uses the same prepackaged code and just modifies it some to better suit their site. If you think that's why they all look very similar then, you have an idea what these are.
It's not true, every menu isn't the same at all despite how they look. That's the dilemma every web designer has now. Do it yourself from scratch or adjust this premade script to suit your needs. Often times I go the easier route. If I'd have done it that way anyway...why bother typing it. I fought this for a while, didn't want to be lazy and all, but now I think it's just a natural evolution of designing sites. Why keep repeating code for the sake of repeating it?
My favorite web designer is Jack Slocum. Over at ExtJS you can see magic. Loads of very ordinary common things you always seen on the web for years, but designed with a flair. It's like turning a honda civic into a ferrari. His work fascinates me. I try to break down the code in a "how'd he do that" sort of way and I always wind up thinking how clever he is. Even how he does a simple common message board is clever. The stats pages that are prettied up XML files are using Jack's API. The game stats page is too. There's a good example to show you what I mean. How many times have you been annoyed with a popup window? Well once you submit that form it's almost a popup but, instead it's called a modal and looks very nice. From popup to nice looking modal (I don't think Jack started modals, just a fitting description/example)
CSS I didn't like to use at first but, you quickly get used to it. I've been designing sites for over a decade and we actually changed the color on every single page. Using CSS, you can change one file and all the pages are updated. CSSZenGarden.com is a clever website for showing people how you can use CSS. Every single page has the same text, every one. However, not one page looks similar as they're all modified with the stylling of CSS.
For AJAX you "have to" go to Ajaxian. It's the center of the AJAX world on the web. Link to cool scripts or ideas, links to good books or video tutorials....if you want something AJAX it's probably there.
CodingForums.com is the advice place. You go there and help others and they help you too. Tons of reciprocal helping makes for a wonderful message board community.
JQuery is a slick library to use. On the main page of this site, at the top are tabs that flow smoothly from one to another. That's jquery.
Prototype is another nice collection of scripts. It's actually outdated but I still love it. It's still very useful.
Adobe has some neat frameworks(oh they're like APIs, UIs, Libraries, and classes too) in Spry and Apollo. Granted they change the name of them from time to time so they might not be called that now but, they're interesting as well.
Layout of webpages in general and templates. As I mentioned with coding the elementary part of scripts, you can begin a webpage from scratch or just go get somebody's template and hack away at that until it's your look and feel to it. There's a billion 3 column layouts for sites on the web. Starting with one just saves time. I do this. Many times someone else has an idea for a page layout(their template) and I think I'm taking it a step further, furtherring their idea. If I leave a page, for the most part, as is then I leave the copyright info in. The menu page here is an example of that. I don't want to rain on anyone's parade.
Hacks- hacks are fun shortcuts. The king of the hacks is O'Reilly. You go over to his site and check out parts of his books. Tons of fascinating shortcuts and different ways of doing things.
Where to learn-Oh start out at HTML Goodies like everyone else. W3Schools is also very good for tutorials. There's an absolute ton of free web design tutorials on the web, you shouldn't have to pay for it.
Well that's me taking a shot at explaining some things for "the layman" and probably a bit too quickly. Hopefully the more savvy web designers don't "beat me up" too much for my layman descriptions.
Thanks for all the questions
I will get back to this and provide a bunch of useful links.
Here's a links page