ACL stands for Anterior Cruciate Ligament is a very common injury to athletes who are into contact sports specifically soccer,basketball, jiu jitsu, and etc. This kind of injury can be a "season ender" for a player because this damages the ligament that prevents your shin bone from sliding out in front of the thigh bone. See image below
Possible causes of having an ACL are changing of direction rapidly, stopping suddenly, landing from a jump incorrectly. The very common sign that you have an ACL is when you hear a "pop" during the injury and you cant bend your knees 90 degrees or even straighten it up. If you are a basketball enthusiast for sure you know who Derrick Rose is. He was at his peak of his career, having MVP award, Chicago Bulls was a contender when suddenly he suffered from an injury which lead all of these into a memory for him. Months passed by he recovered from the injury but it changed his game he was not that explosive anymore, not that high flyer anymore, not that quick no more. After how many games from coming back from an injury he suffered again the same injury but this time it was on the other knee. All of this is because of his quickness, the way he lands after going so high from the rim, and from the way he moves. 
There are 3 stages of ACL.
Grade 1 ACL - Where fibers of the ligaments are stretched, but there is no tear.
Grade 2 ACL - Fibers of the ligaments are partially torn.
Grade 3 ACL - Fibers of the ligaments are completely torn. (This stage needs surgery)
You can prevent this kind of injury by exercising and strengthening your leg muscles, particularly hamstrings, training and exercising for proper techniques and knee positioning in jumping and landing. As an athlete I know this kind of injury could be your worst dream if ever this happens to you. I'm sharing my knowledge about this to help you guys avoid this kind of injury. Right now I'm having grade 1 or 2 ACL which I'm not really sure about it. This injury forced me to stop playing the sport that I really love. I hope after reading this blog you guys will learn something and I hope this would help you avoid from having the same injury as mine.
God bless you all!
Oh my! i don't want that to happen to me. I've been into martial arts last month and experience bruise and other injuries but thank God not to that point. It's important to be careful and safety first. Techniques and good coaches are really important to avoid such things.