An introvert is someone who likes to keep to themselves. Doesn't really like to socialize with others. Thinks before they speak. Often shy.
An extrovert is someone who is outgoing. Likes to socialize with others. Often speaks before they think. Often outspoken.
I am a little bit of both. I am an introvert because I feel more comfortable in a small group of people than with a large group of people. For example, I'll rather have a small dinner with family and friends than throw a big party. I was very shy when I was young. I feel like I have become more an extrovert now as an adult because it is part of my job to work with others. I do socialize with people but sometimes is only when I have to.