What's a Dual SIM Phone?

Dual SIM phones allow both SIMs to be active simultaneously: calls can be received on either number, unlike adapters that make one or other number active as required.

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.

Although battery life (talk time and standby time) of dual-SIM phones is reduced, typically by about a third, when both SIMs are active, to compensate this they often came with better capacity batteries, or in the case of Chinese-made phones, bundle their units with two batteries, compared to single-SIM phones.

Via wiki