I read a rather simple definition in my psychology book (I'm in high school). According to the text, introverts have such a sensitive nervous system that too much sensory (I mean the five biological senses) stimulation makes them fatigued and may overwhelm them. Extroverts' nerves are more "numb" so they need and seek stimulation to feel good.
Shyness, I think, is more like social anxiety.
(I have been very shy and am definitely not shy anymore, but my introversion certainly hasn't disappeared anywhere
)