In terms of computer speed, efficiency and electricity cost, what do these terms used for computers mean?

Raysor and Gary have it down pat pretty much for you, however I want to make sure you understand that your computer's comparison to other computers depends on what your using it heavily. If you compare it to mine, every component is better in mine. but, mine probably costs more money, and I use it for heavy PC gaming. if I was just writing documents or powerpoint presentations, then it would be overkill. yours is perfect for work. but, if your gaming or editing videos, or doing more extreme things, it wouldn't be a good PC for it. It all depends on what your using it for.