DSPs are the electronic hardware chips used to codify and decodify (CODEC) a circuit switched audio stream (TDM) in to packet switched audio stream (RTP) and more. RTP-RTP, transcoding, conferencing, etc.
More info about cisco DSP:
http://www.cisco.com/en/US/prod/collateral/routers/ps5854/product_data_sheet0900aecd8016e845.html
General info about DSP:
http://en.wikipedia.org/wiki/Digital_signal_processor
or
http://www.audiocodes.com/glossary/dsp
A codec is an algorithm to codify and decodify.
An audio codec is specialized in voice or audio information, a video codec in a video information, etc.
An example of audio codecs are G,711, G,729, GSM, G.722, etc. Video codecs are H.264, XVID, MPEG, etc.
General info about codecs:
http://en.wikipedia.org/wiki/Audio_codec
More info about codec and cisco:
http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a00800b6710.shtml
Regards.
