Q: Tube theory

What are the characteristics that make a good electron tube? I understand that the purpose of an "electron tube" is to act as an amplifier or an electronic switching component for high power systems, but what is the ideal composition of one? Schematically they are comprised of a cathode, a grid, and an anode, but what metals, gases, or compound materials are typically preferred for reliability, power output, and having consistent trigger requirements. I've yet to find a resource that clearly states the relationship of the construction and composition of the tube to the performance and operation, any explanation as well as links to resources would be greatly appreciated for my own independent study.