Vespa,
As many in this thread have pointed out, you have to judge an amp by the sum of its parts rather than by by pigeon-holing selected components.
The definitions in the posted URL are a sufficient starting point for beginners to understand the key differences between the different types of amps ..
Personally, I judge an amp by how it sounds like irregardless whether its transistor, tube, hybrid or digital.
I've found tube amps that sound colder than transistor amps and likewise digital amps that sound more tube-like than certain tube amps.
To my ears, a "good" amp should sound warmer as the volume is turned up, .... this is sometimes where small transistor amps tend to sound ice-pick bright.
A "good" amp should be able to cover the middle ground well; the semi-dirty blues or rythem guitar sound.
A "good" amp should be able to respond to different right-hand picking technique and even guitar volume knob settings i.e. .... clean sounding when strummed gently and gritty/ overdriven when you dig into the strings.
Lastly, a lot of times, people make a decision as to whether or not an amp sound good to them based on not just the tone, but also FEEL/RESPONSE. Depending on your playing technique, you may habitually go for amps with a fast transient attack, or perhaps amps where the notes "blooms" with a certain compression "tube sag" or a combination of the two.