How can a computer be characterized from the point of view of its constituent parts?
A CPU (Central Processing Unit) is the brain of your computer, it's like the CPU in your head. It does the actual calculations that make your computer run. The faster this CPU runs, the faster your computer will work.
The GPU (Graphics Processing Unit) takes care of making graphics on the screen, they are very powerful and fast! 
 RAM (Random Access Memory) is where all your programs and data are stored when you're not using them, so if your computer has too little RAM, some programs won't start up or maybe they'll run slowly.
HDD (Hard Disk Drive) stores everything on your computer, including your programs, documents and photos. A large hard drive means more storage for your stuff, which is great!
These are the basic building blocks of your computer and together they make yourcomputer work.