Introvert: one whose personality is characterized by introversion; broadly: a reserved or shy person (Merriam Webster)
Extrovert: one whose personality is characterized by extroversion; broadly: a gregarious and unreserved person (Merriam Webster)
Both the terms introvert and extrovert are largely used to describe dominant personality traits in people. Introverted people are usually seen as very quiet people who keep to themselves, favor an abundance of alone time, and generally do not enjoy late bouts of interaction. Extroverted people are typically seen as exact opposites of introverts. Extroverts are known for being loud, friendly people who love being the center of attention and enjoy talking to everybody. While some of these traits are very true to name, there are others that simply are not fair to generalize in such a way.
Often times it seems like introverts are labeled as "sad people" and that simply is not always the case. I myself am an extremely introverted person, I also consider myself to be an overall happy person. I don't agree with the fact that just because i keep to myself a majority of the time, I have to be sad about it. I enjoy spending time alone, and a lot of the time, I enjoy spending time with other people as well. To me, being introverted is a way for me to keep my sanity, and, in a way, protect myself. I don't feel the need to tell everyone every single detail of my life. I don't feel the need for small, idle chit-chat, it seems pointless, and both participating parties know that it's just an awkward segue until a more interesting, deep conversation comes along. While I can't speak for all extroverts, or give a personal point of view on extroversion, I highly doubt that all, or even most extroverts are all happy people. I've found that much of the time, extroverted people use their extroversion in the same way I use my introversion, as a protective shield. Being boisterous and overly social and friendly can be used as a way to distract from the pain or difficulties in one's own life, and I suspect that it's a good distraction to be surrounded by many people and engage in many conversations.
This isn't to say that introverted people don't like socialization, or that extroverted people can't appreciate alone time. Everybody needs a balance of of social behavior and alone time to keep some form of what we consider to be sanity. I enjoy meeting new people, even though the concept of talking to someone I do not know and am not comfortable with makes me want to throw up a little bit. Sometimes it can be difficult even to tell whether someone is introverted or extroverted, but I'll let you in on a secret: it's okay to ask. Knowing this one little piece of information about somebody is often key to understanding a big part of them. It might be the reason someone ignores your texts and phone calls, or it might be the reason it seems this person is always blowing up your phone trying to make plans.
The point is, while these personality traits are a big part of most people's lives, they are not a defining factor. Many behaviors can be attributed to them, but nobody should be held to the stigma of their personality. So go make a new friend, whether they come up to you babbling a mile a minute, or if you have to pry their nose out of a book.
