Crossfit has been growing popular in gyms and with tactical personnel recently.
Tactical personnel (soldiers, police, fire) are given most of the credit for the boom in Crossfit affiliated gyms in fact.
Crossfit mixes Olympic lifting, power lifting, calisthenics, gymnastics, sprints, and plyometrics together in workouts called WODs. WOD stands for workout of the day, and have names like Cindy and Fran.
Greg Glassman founded Crossfit in California. He does not believe in setting goals, instead the idea is to be fit for anything.
The workouts are typically a hard 15 minutes of intense work with a group of people that offer copious amounts of positive encouragement to each other.
All ages and fitness levels are welcome because the idea is that anyone can do the WOD — it is just scaled to their level.
This is where the good idea can turn to a bad one. The scaling and exercise selection are what take most of the criticism.
Some of the lifts used are not ideal for use under fatigued conditions, such as plyometrics, power lifting and Olympic lifting.
These types of exercises are technically demanding and error can cause injuries and may reinforce poor technique. Also, the group mentality can push people to injury unintentionally, but some people have even been known to take it to an extreme.
Rhabdomyolysis (Rhabdo) is a condition of muscle breakdown so severe that it can cause kidney damage or failure. This is typically seen in crush injuries. This happens in fringe groups of Crossfit and is mocked with a cartoon mascot, Uncle Rhabdo.
Like anything in life, extremes are rarely good.
The take-home would be to do body weight exercises in a circuit fashion for 15 minutes and do not injure yourself.
If you do this, you will have a great fat-burning workout that will also build muscle, just make sure you change up programs and intensities.