There are two major types of centerfire rifle cartridges available on the market today:

Those which are loaded with steel, and
Those which are loaded with brass
This seemingly simple variation has caused a never ending stream of argument, discussion, speculation, and questioning from new and seasoned shooters alike. Complicating the conversation are other variables that typically get lumped into the argument without proper segmentation, such as:

The different coating options available on the steel-cased ammo (lacquer or polymer)
The different projectile loadings available (copper jacketed lead, the bi-metal coating that most Russian manufacturers use, etc)
The different propellant (gunpowder) burn rates.

http://www.luckygunner.com/labs/brass-vs-steel-cased-ammo/