The term codec is an combination of encoding and decoding. An codec makes audio signal smaller so that it can be transmitted well and stored easily. During decoding, the audio signal is restored again.
There are hundred of audio codecs out here. The question is, which shall be used.
For transmissions in the Internet, if Internet packet losses occur, an codec that support packet loss concealment is useful. Opus and Fraunhofer AAC-Codecs might be a good choice.
The coding efficiency of those modern codecs is quite well. A bit rate of 64 kbit/s is sufficient for a mono signal and 128 kbit/s for a stereo signal.
Traditionals, audio codecs support codecs like G.722, MP2, MP3, and APT-X. However, we do not recommend to use them on the Internet – they are just good for ISDN.
Opus has a good Forward Error Correction (FEC) which makes him well suited for the Internet. In addition, he is supported by those billions of web browsers that support WebRTC. Thus, Opus is the first chose if you plan to use WebRTC or the public Internet.