We are working with three different, totally separate methods.
1) MARINATING
When marinating something, you are adding different ingredients to the original product that is modifying the original through a acid/base method to enhance the original. For example, you take sliced cucumbers. Add some lemon juice for acid, counter with a base of corn syrup for sweetness, and now you have marinated sweet/sour cucumbers. You can marinate vegetables, fruits, meats, poultry, fish, etc. with almost any combination of ingredients to enhance or counter its natural flavors.
2) BRINING
Brining is a method of adding salt to meat, fish and poultry along with other similar ingredients to enhance it's flavoring through osmosis, which simply means you are achieving a new equilibrium of meat and salt combined. This adds a preserving method to the meat with added salt. Salt is a flavor enhancer. It also adds moisture into the meat Therefore, brining adds more flavoring via the addition of salt. Of course, other ingredients can be added, but it does NOT change the composition of the meat itself.... a brined chicken leg is still a chicken leg that is saltier.
3) CURING
The last step is curing. A curing agent ALTERS the composition of the meat (poultry, fish, etc.) by adding sodium nitrite to the meat. (You can research all the different variants but today's accepted ingredient is 6.25% sodium nitrite in volume). The most common method is adding it with salt in a brine. Then you cure, or pickle, the meat for a time, then smoke it. What you achieve is a cured and smoked product that has the characteristics of hams, bacons, etc. that classic familiar flavor we all love!
You can brine AND cure chickens and turkeys for smoking, but this alters the poultry into a cured and smoked product with similarities like ham with that hammy flavor. A cured and smoked turkey is a delicacy to truly enjoy!
There's a lot of other things that occur when smoking meats too, but to help clarify these particular processes and their differences this should help!
