Freezer bag cooking usually means cooking food at home, dehydrating it, and combining with selected ingredients in single portions in a freezer bag. On the trail, you add boiling water to the bag, let sit 10 minutes or more (works best in a cozy of some sort to keep the heat in) and then eat from the bag - no muss, no fuss. It works very well but does have its limits. Instant foods are a snap but pasta and rice must be precooked and dried and some dehydrated fresh foods are a challenge, even for an excellent cozy.
On a trip last Fall, I took a vacuum food jar as a cozy. It can keep things hot for hours if needed and has a lid that seals, so you can put a meal on whenever it is convenient and then eat when you want to. It can keep things at cooking temperature longer than a cozy, so you can buy more ordinary staples and depend less on processed foods or dehydrating pre-cooked food ahead of time.
The food jar worked well, but was not up to cooking conventional rice and would not stay hot overnight. After trying several things, I settled on an ordinary (narrow mouth) vacuum bottle. It turns out most of the heat is lost through the lid, which has foam rather than vacuum insulation. The shape and narrow mouth are a problem for the freezer bag, but it turned out to be no problem to cook directly in the flask. I pack meals in sandwich bags, empty them into and cook directly in the flask (and reuse the bags). I use a long handled spoon and eat from the vacuum bottle. The flask weighs more than a cozy (I used a 0.5 liter REI Bullet, at 9 ounces) but not a lot more and there is no wet food bits in used freezer bags to pack out. When you are not cooking, you can use it for hot or cold beverages or to, for example, smuggle a pint of Ben and Jerry's out to the town.
A more complete description of this is at
http://www.crazyguyonabike.com/doc/?doc_id=2350
This trick is not for everyone, but if you are doing a long trip and resupplying in trail towns, it may save you from living on ramen.
