Dual SIM Active Phone:
This kind of dual SIM mobile phones have two transceivers and naturally consumes more battery power. The two transceivers independently work with the two SIM cards and is available to be called any time when the other SIM is busy. Thus, when a person is talking on SIM-1 for a long time, a call incoming in SIM-2 will not get busy or unreachable tone.
I have said about Samsung, examples of this kind of phone models are Samsung B5722, Samsung C6112, Samsung C5212 etc.
Dual SIM Standby Phone:
This kind of dual SIM mobile phone works on a single transceiver and handles both SIM cards at a time. In standby mode, both SIM cards remain on standby, ready to receive calls. But when a person is on a call in SIM-1, the SIM-2 gets automatically cut off until the call on SIM-1 ends. Which means, during any incoming/outgoing call, only one SIM is active and the other is briefly unreachable.
If you take a close look at the display screen’s signal tower icon, you’ll see signal on one SIM is showing and signal on the other SIM is not displayed or disabled while one SIM is handling a call. Examples of this kind of phone models is Samsung E1252 or Nokia X1-01 etc.