A container or wrapper is a file format that specifies how different streams of data can be stored together, or sent over a network together. It allows audio and video data to be stored in one file and played back in a synchronised manner. It also allows seeking in the data, by telling the playback software where the audio and video data is for certain points in time.
In addition to audio and video, containers may provide meta data about the data they contain, including the size of the frames, the framerate, whether the audio is in mono or stereo, the sample rate, and also information about the codecs used to encode the data.
When you play a digital movie that has sound, your player is reading the container, and decoding the audio and video using separate codecs. Theora video is usually stored or streamed together with Vorbis sound in the Ogg container, but it can be stored in other containers too. Matroska (.mkv) is another format people use for Theora video.
The difference between containers and codecs.
The three letter extension at the end of the file name refers to the container, not the codec. People often get confused about this. When a file ends in .mp4 or .avi, those are containers that could contain several different combinations of audio and video streams. Certain containers don't work with certain codecs, and certain codecs work best with certain containers. But you can't tell for sure what codecs a video file requires by looking at the file extension.