Introverts are more concerned with the inner world of the mind. They enjoy thinking, exploring their thoughts and feelings. They often avoid social situations because being around people drains their energy. This is true even if they have good social skills. After being with people for any length of time, such as at a party, they need time alone to "recharge."
When introverts want to be alone, it is not, by itself, a sign of depression. It means that they either need to regain their energy from being around people or that they simply want the time to be with their own thoughts. Being with people, even people they like and are comfortable with, can prevent them from their desire to be quietly introspective.
Being introspective, though, does not mean that an introvert never has conversations. However, those conversations are generally about ideas and concepts, not about what they consider the trivial matters of social small talk.
Introverts make up about 60% of the gifted population but only about 25-40% of the general population. Source
If a crowded cocktail party feels like a holding cell to you, even as you gamely keep up your end of the chatter, chances are you're an introvert. Introverts are drained by social encounters and energized by solitary, often creative pursuits. Their disposition is frequently misconstrued as shyness, social phobia or even avoidant personality disorder, but many introverts socialize easily; they just strongly prefer not to. In fact, the self-styled introvert can be more empathic and interpersonally connected than his or her outgoing counterparts.