As you have figured out by now, there are many different types of electronic components, and you must be familiar with all of these. They all act differently with reference to voltage, current, temperature, pressure, and other outside influences. In order to make learning electronic components easier, they have been divided into two categories:
While possibly not the best definition, the key difference between active and passive components, is that active components have the ability to produce gain, or amplify a signal, and passive components do not. Some would argue that a component's ability to switch a signal makes it an active component, but I don't see a toggle switch as being active. I may modify this definition later, but for now, this one is enough for you to grasp the concept.
So far, all the components we have discussed are resistors, capacitors, and coils. These are passive components. Now we are going to begin learning about active components.
Some examples of Active components include Vacuum Tubes, Transistors, Integrated Circuits, etc. We will first study Vacuum tubes, as they are a fundamental building block in the understanding of other active components.
Many "modern" schools today are skipping right over tubes. I plan on EMPHASIZING them, as I see them as still a very viable and cutting edge technology. There are new tubes being developed and used every day, because up 'till now, we simply haven't found a device which is more capable of linear amplification at high power and high frequency levels. Some examples would be the klystron, magnetron, Inductive Output Tube (IOT), Traveling Wave Tube (TWT) et al. There have also been leaps and bounds in nanotube technology, and lasers still use tubes as well.
I'll be willing to bet that you have at LEAST 1 vacuum tube devices that you use on a regular basis in your home right now! You probably cook meals in a Microwave Oven, which uses a magnetron. In some cases, your TV or possibly your computer monitor may also have a Cathode Ray Tube (CRT). And should we, someday, find a way to replicate food or transport people as in "Star Trek", I believe it will be first developed using technology similar in nature to vacuum tubes.
Now some might say that these are the exception, not the rule - that the majority of electronics jobs will never require tube knowledge. I concur, but submit that if you want to make the big bucks - you have to be a specialist, and specialists deal in cutting edge technology - many of which require tube knowledge. Satellites going into orbit still typically use Traveling Wave Tubes, not transistors for their main power amplifier stages.