The SELCAL is used for communications. The transponder is used to identify the aircraft on radar, and provide some information (depending on the mode of the transponder). Whereas the an aircraft will have a distinct SELCAL code, the transponder is a device where you select the code (usually on instruction from ATS, but there are some specific reserved codes, such as for emergencies, and flights outside controlled airspace). The transponder code is made up of 4 digits, each 0-7 (giving a total of 4096 distinct codes).
"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh