There are a great many different types of vacuum tube in existence, however it is possible to group tubes according to some fairly basic criteria. There are two primary distinguishing features, the source of free electrons within the device and the gaseous filling (or lack of it) within the tube envelope. The later of these two concepts we have already introduced by implication. A vacuum tube is a device with a vacuum (very low pressure gas) filling. And a gas filled device is, as the name would suggest, filled with gas that might be at a pressure somewhat above or below atmospheric. The type of gas used is also an important feature, particularly in switching tubes where a wide variety of fillings are encountered.
The source of the free conduction electrons in the device may be either thermal such as a heated filament physically associated with the cathode of the device - a hot cathode, or alternatively a simple consequence of a high voltage gradient across the device, resulting in autoemission from the cathode. A device employing this latter method is known as a cold cathode device. In high voltage switching the presence of high voltages, and hence the possibility of large voltage gradients within devices means that the cold cathode system, quite a rarity in most other types of tubes, is the norm rather than the exception.The delay time is the time taken between the application of a trigger pulse and the commencement of conduction between the primary electrodes.
Jitter is the variation of time delay from shot to shot given similar electrical stimulus.
The commutation time is the time taken for the conduction to reach maximum once it has commenced. (i.e. From the time from the end of the delay time to the time at which the maximum level of conduction occurs.)
It should be pointed out that none of the switching tubes we are about to consider look very much like the things in the back of an old radio set. Many are large, some exceptionally so. Also glass has largely given way to ceramic in the higher powered devices. Before you go down your local electronics shop or radio shack it should also be pointed out that many of these devices besides costing $100's (often $1000's) a piece, and are also largely unavailable to the general public due to their application in advanced missile and nuclear weapon technologies. Of these devices the most 'everyday' is the ignitron which finds much application in industrial welding situations.