I don't really understand what to do with my description and keywords meta tags. Where do they go in my web page, and what should I put in them? Are they necessary? No one sees them, right?

You are correct. They are not necessary, and no one sees them. No one, that is, except search engines. And some search engines very carefully consider the description and keywords when evaluating where to place your page in search engine results. So let's talk about them. First, your metatags go in the 'head' of the page, which means you should put them somewhere between the 

and

 lines. The two tags should look something like this:

Description

Search engines use description meta tags in two ways, too varying degrees:

1. Occasionally search engines will use your description tag in their display of information about the page, when your page comes up in a search result. This is the only time an actual human reader is likely to see your description. Thus, it ought to be something that is human readable and comprehendable, and - if possible - enticing to the reader to click the link. For example, if you go to Google and search on the phrase "secret word" one of the highest ranking web pages is our Secret Word Game. The description under the link on Google's search results is "Secret Word - Guess letters from the alphabet and determine the computer's secret word." Where do you suppose Google got that text? That's right - it's the meta description tag on our game's web page.
2. Search engines look at the description tag and take not of any important words that show up there; they may help the search engine determine what your page is about, and therefore how to rank it for various search queries. Thus, it's important that your description meta tag contain key information related to the page's topic.

Keywords

The name 'keywords' is deceptive, because keywords are not nearly as helpful as key phrases.  Fill your keywords metatag with phrases you think people might use when searching.  having "webpage design" as a keyword is much better than having "webpage" and "design" as two separate keywords.

Be careful not to repeat words or phrases too much, however, as some search engines will penalize you if they find the same word more than three times in your list.  

2016 Update: It appears that search engines are relying on Keywords less and less when it comes to ranking search results. There's a very good reason for this. Unlike the page title, which appears both in the browser title bar, and the search engine links, and unlike the page description, which also appears in search engine results, the keywords don't appear in a human-readable place at all. This means that dishonest web developers felt comfortable using the keywords as a place to try tricky methods to make their web pages rank higher. Since they're easily spammable, search engines are relying on them less and less. I usually tell my clients to put in a few keywords and phrases that pop quickly to mind, but don't spend too much time sweating over it. Craft good content, and let it speak for itself! 

One Last Piece of Advice

Make sure that the key words and prhases you use in your metatags also show up in the text of your page.  No search engine is going to be impressed if your page title, description, and keywords all have the phrase astronaut training program, but that phrase doesn't show up once in your page content.  When it comes down to it, though the metatags are important, what's actually on the page is the most important of all.

My son loves browsing the internet, and he's found a message board he wants to join. I'm uneasy about letting him do it, partly because he has to give his email address, and other information. What are your recommendations on children joining websites?

First things first. Somewhere on the website your child wants to join, there should be a link with a title something like "Terms Of Use" or "Privacy Policy".

In theory, this link is prominently located on the home page, but that is not necessarily the case. You may need to search to find it. In some cases, the Terms of Use (TOU)/Terms of Service (TOS) and Privacy Policy won't even appear until you start registering. Not a problem; you can start the registration process and then cancel it if you don't like the usage details.

Read through the information carefully - especially the Privacy Policy. If the site caters to children, it is required to tell you specifically what information it will collect from your child, and what purposes it will use that information for. You should read and consider this before you let your child join.

Next, you should consider running a search for the website, to find out what other people think of the website your child wants to join. If you are satisfied that the site appears to be reputable, and you are comfortable with their TOU and privacy policy, then walk through the registration process with your child, making it clear that your child is not to join other websites without your approval. As I said in another post about children and the internet you should try to make the internet a family experience.

Dear Professor Puzzler, My son is always pestering me to go online, and I am very uneasy about letting him browse the internet, because I know there's some really trashy websites out there. Have any suggestions?

The internet can be a frightening place.

It's like a city--there are places that are filled with bright lights and dazzling and exciting attractions. There are beautiful parks, museums, and zoos. There are also slummy, dirty, dark places where you wouldn't go in broad daylight. How much do you let your child wander the city alone?

You should be even more concerned about letting your child wander the internet alone, because in some cases the difference between the brightly lit park and the slum district is one click. I can go to google.com, type in one search term, and have a plethora of inappropriate websites (hate sites, pornographic sites, etc) listed on my computer screen.

Browse the internet with your child. Help your son find the brightly lit places, and make the internet a family experience.

As your child grows older, he will begin having more freedom--both at school and at home--and the best thing you can do for him while you have the opportunity is to teach him appropriate use of this powerful (yet oh-so-frightening) resource we call the internet.

2016 Update: Internet use has become ubiquitous among children in the decade since this was written. Everyone seems to have a smart phone, and parents tend to forget that the same resources (good, bad, and ugly) that are available on the home computer are also available on the phone. The warning to proceed with caution is as important as ever!