Your computer has a processor chip inside it that does the actual computing. It has internal memory (what DOS/Windows people call ``RAM'' and Unix people often call ``core''; the Unix term is a folk memory from when RAM consisted of ferrite-core donuts). The processor and memory live on the motherboard which is the heart of your computer.
Your computer has a screen and keyboard. It has hard drives and floppy disks. The screen and your disks have controller cards that plug into the motherboard and help the computer drive these outboard devices. (Your keyboard is too simple to need a separate card; the controller is built into the keyboard chassis itself.)
We'll go into some of the details of how these devices work later. For now, here are a few basic things to keep in mind about how they work together:
All the inboard parts of your computer are connected by a bus. Physically, the bus is what you plug your controller cards into (the video card, the disk controller, a sound card if you have one). The bus is the data highway between your processor, your screen, your disk, and everything else.
The processor, which makes everything else go, can't actually see any of the other pieces directly; it has to talk to them over the bus. The only other subsystem it has really fast, immediate access to is memory (the core). In order for programs to run, then, they have to be in core (in memory).
When your computer reads a program or data off the disk, what actually happens is that the processor uses the bus to send a disk read request to your disk controller. Some time later the disk controller uses the bus to signal the computer that it has read the data and put it in a certain location in memory. The processor can then use the bus to look at that memory.
Your keyboard and screen also communicate with the processor via the bus, but in simpler ways. We'll discuss those later on. For now, you know enough to understand what happens when you turn on your computer.