whats the difference between gsm and cdma ?

They are different technologies.. basically on type of technology it uses for allocating channels to the users.. GSM network uses TDMA i.e. it will allot a particular time to each user.. whereas CDMA will allocate a user a particular code (in CDMA the c stands for CODE) usually the specific code is installed in the mobile phone you buy.. this is why cdma phones are not usually available without SIM card..

Well, technically, you have 2 different questions here. GSM stands for Global System for Mobile communication. CDMA stands for Code Division Multiple Access. GSM also uses Subscriber Identity Module (SIM card). If your phone has SIM card in it, chances are, it's GSM phone.

According to my knowledge the Nokia E61 is a GSM phone. In my country, GSM and CDMA card can be distinguished clearly, because the numbers are very different.

I am a telecommunications engineer here in the Philippines and based on my experience with Nokia E61, the 3G sign in the signal bar is one indication that you are using wcdma2100. E61 phones can accomodate GSM and WCDMA as long as there is a network service in your country with frequency bands of 900, 1800 or 2100. When there is a 2G signal present, the signal bar will not have a 3G sign under it. I hope this helps.