The Shape of a Computer
Almost every modern computer follows the von Neumann architecture: a central processing unit (CPU) connected to a single main memory that holds both the program's instructions and its data, linked by buses.
The CPU does the work. Inside it, the control unit fetches and decodes instructions and directs the rest of the machine, while the arithmetic logic unit (ALU) performs the actual computations — additions, comparisons, logical operations. The CPU also has a small set of very fast storage locations called registers for the values it is working on right now.
Main memory (RAM) is a large array of numbered cells, each holding one byte, identified by its address. Memory is volatile — its contents vanish when power is lost — and it is far larger but far slower than registers.
The buses are the shared wires that move information between components. Conceptually there are three: the address bus (the CPU puts an address on it to say which cell it wants), the data bus (the byte travels here, in either direction), and the control bus (signals like "read" or "write" that say what to do). The width of the address bus limits how much memory can be addressed: address lines can name cells.
Common pitfall: believing programs and data live in separate memories. In the von Neumann design they share the same main memory — a program is just data that the CPU chooses to interpret as instructions. This unification is powerful (programs can even modify programs) but also the root of whole classes of security problems.