What is "E30 M50"?

BMW uses codes such as E30, E36, or E60 to identify their vehicles. This is a convenient way to identify a BMW because a 325i could have been made in 1987, 1994, or 2004.

E30
An E30 is a 3-series made from 1984-1992 (give or take a year depending on model/country).

Likewise, BMW uses codes to identify their motors. These have an M like M20, M30 or M50 or an S like S14, S38 or S52. The difference being that S motors are found in M cars like an M3.

M50
An M50 is the dual overhead cam, inline 6 cylinder motor found in E36 models (1992-99 3 series). I have the M50TU version which means that it also has VaNoS (variable valve timing).

E30 - M20 + M50 = Ultimate Driving Machine
My E30, a 1986 325es, originally came with an M20 or more specifically a M20b27 because it is a 2.7L M20. This motor is commonly referred to as 'eta' and was designed to rev low (5000 rpm), provide lots of torque and decent fuel economy.

I decided that I needed more power than the 120HP and 170 ft-lb of torque so I swapped in a M50 motor which makes 192HP and 181 ft-lb of torque. Then end result is more power, better fuel economy, reduced emissions and improved reliability. It is easy to see why this swap has become so popular in recent years.