A "B Ration" is one that is from a can, or is preserved some way. some of us remember canned bacon, steaks in cans (just add water) and so on. These are generally served in the field mess facility. No refrigeration required.
"C-Rations, D-Rations, Iron Rations, K-Rations and so on, were given to the troops for them to fix. The "C-Ration from World War II fame (and still around from Korea) was a single can, everything was in it, including several small cans. The K/Iron/D rations came in a "cracker jack box".
The Meals Combat Individual (MCI) came out at the start of the Kennedy Administration (they had been developed but they were waiting for the WWII stocks to go down). This was the "C-Ration" for the folks in Viet Nam and what a lot of us remember up to 1982/83.
The Meals Ready to Eat (MRE) has been through several variations as has the infamous "T-Rations".