What is "Blow-Out" ?

The hydrocarbon reservoirs often contains oil and gas naturally in high pressure. A lots of precautions needs to be taken to contain the high pressure in designing a oil or gas well and similar precautions are taken during drilling and then producing the well.

However, due to equipment failure or human error, if the flow of reservoir fluids to the surface is becomes uncontrollable, it is called "blow-out". Inflammable fluid (oil+gas) often catches fire and damages rigs and other surface equipment.

During drilling, often gust of high pressured fluids enter the well. The phenomena is called "kick". Controlling the kicks are special skills the drillers are expected to learn and implement. 

BOP or "Blow Out Preventor"s are special equipment used during drilling and work-over to prevent such events.