A codec is a method of encoding or decoding data. more such just a way for people to compress data to make it easy to use,store,etc.
A container is what holds a grouping defined by the codec, say you had a ton of files organized by the codec, after they where compressed, decoded, encoded, etc. you would place them in the container. it pretty much packages, transports, and gets everything set up for you.
Anthany Campbell 