I brought in a computer and we did a quick disassembly of it while I tried to explain what each main part was. I've tried to duplicate what we covered at this page (lots of info here! and pics!). I also handed out an introductory sheet that covered some of the basics.
I reviewed computer components, and that will most likely be the last day for that. If you want to learn more about components, I noticed that there is a fairly good introduction at The PC Guide. We also started talking about binary formats for information. I showed a table of ASCII to binary mappings. Because ASCII is a standard format for text there are tables all over the web . (But don't worry, they all say pretty much the same thing.)
You may have noticed that there are actually more characters than what is included on all of those ASCII tables. Notably, Japanese has something around 4,000 symbols. That is why there are other character encodings than ASCII (but we didn't cover them in class). Here is a brief summary that you should read. UTF-8 is another similar standard. If you pay attention to text encodings, there is a rumbling going on as the computerized world shifts from ASCII to Unicode. ASCII has been the standard for decades, but now people are trying to phase it out.
Underlying all of this is binary. We talked about simple counting integers in binary. (Here is another tutorial.) Always keep in mind that hexadecimal is simply another way to represent numbers. Hex is commonly used to represent binary because a set of four binary digits equals one hex digit. Decimal (base 10, what you learned in grade school) isn't as easy to convert back and fourth between binary. Additionally, standard computer number formats such as bytes, words, shorts, integers, and longs all split evenly into hex.
I noticed that we had a few minutes, so I introduced the idea of floating point. I'll warn you here that it is much more complicated than integer binary counting, but it is also very widely used. I don't expect you to study floating point, but if you are interested in computers you will have to understand it the sooner the better. (You may have noticed by now that I post many links related to the same thing. This isn't because you should read all of them. Go through and find at least one that you understand. Read that one. After that you will be able to understand more interesting pages that I linked to. And you can read another, and another! Weeee!)