In amateur radio, QRP operation means transmitting at reduced power levels while aiming to maximize one's effective range while doing so. The term QRP derives from the standard Q code used in radio communications, where "QRP" and "QRP?" are used to request, "Reduce power," and ask "Should I reduce power?" respectively. The opposite of QRP is QRO, or high-power operation. Most amateurs use approximately 100 watts of power, and in some parts of the world can use up to 1500 watts. QRP enthusiasts contend that this isn't always necessary, and doing so wastes power, increases the likelihood of causing RF interference to nearby equipment etc.
There is not complete agreement on what constitutes QRP power. While most QRP enthusiasts agree that for CW, AM, FM, and data modes, the transmitter output power should be 5 watts (or less), the maximum output power for SSB (single sideband) is not always agreed upon. Some believe that the power should be no more than 10 watts peak envelope power (PEP), while others strongly hold that the power limit should be 5 watts. QRPers are known to use even less than five watts, sometimes operating with as little as 100 milliwatts or even microwatts! In brief: QRP operation is divided in three categories according to the used power. QRP means power 5 to 1 watts, QRPp means power below 1 watt to 100 milliwatts and QRPpp means any power below 100 milliwatts.
Communicating using QRP can be difficult since the QRPer must face the same challenges of radio propagation faced by amateurs using higher power levels, but with the inherent disadvantages associated with having a weaker signal on the receiving end, all other things being equal. QRPers try to make up for this through more efficient antenna systems and enhanced operating skills. QRP is especially popular with CW operators and those using the newer digital modes. PSK31 is a highly efficient, narrow-band mode that is very suitable to QRP operation.
Many of the larger, more powerful commercial transceivers permit the operator to lower their output level to QRP levels. Commercial transceivers specially designed to operate at or near QRP power levels have been commercially available since the late 1960s. As QRP has become more popular in recent years, radio manufacturers have introduced radios specifically intended for the QRP enthusiast but the majority of QRPers prefer to construct their equipment from kits or homebrew it from scratch.
 Wikipedia, QRP operation, http://en.wikipedia.org/wiki/QRP_operation