Buddhist Sects
Buddhism is a world religion and the different sects came about, in theory, to integrate the religion to meet societal and cultural needs of the different areas that Buddhism really took hold in.
Historically, it is not known exactly how or why these sects first started.
Be careful not to get too wrapped up into the divisions in Buddhism that you lose sight of the trans-cultural impact that Buddhism had throughout the Asia.
Now when some people say the different sects of Buddhism, I think they tend to be thinking of the different cultural Buddhisms. However, what the different sects represent is the different schools of thought.
I pulled the following list from
http://www.gkindia.com/worldreligions/themajorsects.htm . Because is was the most accurate and simple. I added some on my own notes within the post to help simplify it further. Apart of the list for each school of thought is the region or country that follows it. For example the Mahayana Buddhism goes with Chinese Buddhism. While the Vajrayana goes with Tibetan Buddhism.
The Major Sects of Buddhism
Theravada (Hinayana) Buddhism - Enlightenment just for the self
Literal: School of the Elders (Small Vehicle)
Main Focus
Four Noble Truths
Meditation
Sage Buddha
Main adherents: Southeast Asia
Mahayana Buddhism - spreading Enlightenment to others
Literal: Large Vehicle
Main Focus
Four Noble Truths
Meditation
Divine Buddha
Bodhisattvas
Main adherents: China, Japan, and Korea
Subdivisions
Pure Land School
Tian Dai (China) or Tendai (Japan) Buddhism
Confucianism Influence
Chan (China) or Zen (Japan) Buddhism
Daoism Influence
Main Focus
Meditation
Chanting
Teacher-Student dialog
Vajrayana Buddhism
Literal: Diamond Vehicle
Main Focus
Meditation
Chanting
Enlightenment in one lifetime
Tibetan gods and demons
Religious visualizations
Philosophical debate
Ritual
Yoga
Tantric Sex
Main adherents: Tibet
Beneath all these sects are lineages and schools and under those schools are more schools of thought.