What is blow-by?

Blow-by is the leaking of combustion gases past the piston rings and into the engine block. All engines have some blow-by, since piston rings can never seal perfectly. Most cars have a vent (the positive crankcase ventilation valve, or PCV valve) which lets these gases out of the engine block while keeping (most) of the oil in.

Read Jack's Transmissions PCV article. 

On DSMs, the PCV valve is connected via a hose to the air intake. Since oil leaks out of the PCV valve, oil gets into the intake and usually gums up the inside of the intercooler. Many DSMers remove the connecting hose to correct this problem. To prevent oil spraying into the engine compartment, they also install either a filter or a catch can [[What is a catch can?]] in place of the PCV valve.