Obviously, deep knowledge of electronics requires many years of study and consumes the professional life of electrical engineers. However, a working knowledge of electronics can be acquired by just about anybody who is motivated. Although the never-ending process of miniaturization continues to make the workings of electronic components more abstract (unlike seeing the glow of a working vacuum tube), the wide availability of parts, low cost, and huge amount of "how-to" information on the Net, more than make up for it.

So, where does one start? Although I avoid promotion of commercial entities, in this case I make no apologies: Radio Shack. Yes, there are cheaper sources, and certainly the selection of parts is limited, but despite how unprofitable this portion of their business must be, you can walk into any of their stores in just about any town, and take your pick from a wall of resistors, transistors, IC's, and more. Amazing. Plus, they stock excellent beginner's books--I especially recommend anything by Forest M. Mims III. To build a working knowledge of electronics, you will need a solid understanding of Ohm's Law (requires grade school algebra). And that's about it.

Most of the practical electronics necessary to get information from the PC to a machine boils down to choosing between "you got 5 volts" or "you ain't got 5 volts." A crucial benefit here, is that although you can readily fry parts by screwing up, you won't die from electrocution when working with so small of a voltage. There are many ways to learn basic digital electronics. A few of my favorites include:

Nuts and Volts magazine: I absolutely love this monthly rag. It is written for non-engineers, but packs a tremendous wealth of practical know how between its covers.

Radio Shack's various experimenters packages and books.

If you're getting serious, and want deeper and a more authoritative reference: The Art of Electronics, by Horowitz and Hill, is it.

From Bits, to Bytes,... to Bots: a course I have taught to kids as young as 12, serves as an example of one strategy for doing elementary digital electronic experiments, involving connection of a PC to the real world. See this if you want more specific information about motion control circuitry.