We know that you don't always have time to watch a long video or read a huge wall of text or notes.     Here are the important points from Jeff Kaplan's breakdown of competitive play in Overwatch. 

Jeff Kaplan’s TL;DR

• Beta feedback was that competitive play was not competitive enough. 
• Seasons will be three months long.   Based on real world seasons in the Northern Hemisphere. 
• Play for about 2.5 months, break and then new season. 
• Games went into Overtime too often; 35% of the time.
• Changes in place to minimize Sudden Death.
• Sudden Death will resolve on the map you are playing, not on a random control point map. 
• Assault maps flipped too quickly; changes to allow for more back and forth. 
  • Details to follow.
• Progression vs Skill; in beta it leaned towards progression.  
  • Could not be demoted out of a tier.  
  • There will be no Tiers.   
• MMR will be correlated to “Skill Rating”; 1-100 rating. 
  • You will be showed your Skill Rating
  • In game all Skill Ratings will be shown and team average will be shown. 
• Underdogs can gain more and lose less. 
• Also shows if teams have parties in them. 
• If there are issues the system will continue to evolve. 
• Rewards; cosmetic rewards (sprays, icons, customized “golden gun” system)

If you would like to watch Jeff explain competitive play then here you go!