GSM and PCS

What is the main difference between GSM and PCS technologies?, which one is better, and why?, anybody could help me?

A good way to ask is to search google.com first, and after you read the documents, those terms or statements you cannot understand, you can ask here.

Actually, they are more or less the same in protocol layer, the difference is RF physical layer, since they are in different freqency band, by the way, there are also many standard about GSM, like GSM850, GSM750, GSM450, EGSM900 and RGSM, PGSM,... for PCS, only PCS1900, also DCS1800.
We could not say which one is better, since they are ultilizing time mupltiplexing method to transmit or receive data.

E.g. EGSM900, in Hong Kong, only 3 operators, because 30MHz BW for Uplink, 30MHz BW for downlink, each operator occupy 10MHz, with 200kHz channel BW.
On the contrary, DCS1800, in Hong Kong, 6 operators, 60Mhz BW for Uplink, 60MHz BW for downlink, exactly the same as EGSM900

They are almost the same except number of channel and operating frequency.

GSM: Channel 0-124
EGSM: Channel 975-1023
PCS : Channel 512-885