kongas,
diplexer = duplexer = putting two signals with very different frequency bands onto the same feeder.
For instance : Tx and Rx signals of a TRX GSM900 onto one feeder.
And then the Tx/Rx of GSM900 with Tx/Rx of DCS1800.
combiner = putting two signals with similar frequencies onto one feeder.
For instance each Tx of 2 TRX GSM900 onto the same feeder. Both frequencies are so close to each other (< 100 MHz apart) that a combiner is needed.
Combiner introduces more than 3dB loss
Duplexer introduces less than 0.5dB loss.