A 
dual SIM mobile phone is one which holds two SIM cards.  Initially, dual-SIM adapters were made available to use in regular  mobile phones to allow them to contain two SIMs, and to switch from one  to the other as required. This combination is called a 
standby dual-SIM phone.
 More recently, some phones have been produced that can natively work  with two SIMs, both of which may be active at the same time. These are 
active dual-SIM phones. There are also some Chinese triple-SIM phones. Akai Trio is the first phone with 
triple GSM SIM slots.
 Dual-SIM operation allows the use of two services without the need to  carry two phones at the same time. For example, the same handset can be  used for business and private use with separate numbers and bills; or  for travel, with an additional SIM for the country visited. Using  multiple SIM cards allows the user to take advantage of different  pricing plans for calls and text messages to certain destinations as  well as mobile data usage.
 These phones have until recent years been largely eschewed by the  larger phone manufacturers partly due to their close ties with mobile phone networks who would prefer that customers use only one network exclusively. However more recently manufacturers such as Nokia and Samsung have started producing these phones, which will target customers mostly from developing countries.
- Dual Sim Standby (DSS) - allows the device to switch from one network to the other when required.
 - Dual Sim Active (DSA) - allows the device to be connected to both networks at the same time.
 
Source: Wikipedia