QRP
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.
Philosophy
Most amateurs use approximately 100 watts of power, and in some parts of the world like the US, 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 interference to nearby televisions, radios, and telephones and, for United States' amateurs is incompatible with FCC Part 97 rule, which states that one must use "the minimum power necessary to carry out the desired communications."
Practice
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).
QRPers are known to use even less than five watts, sometimes operating with as little as 100 milliwatts or even less. Extremely low power—1 watt and below—is often referred to by hobbyists as QRPp.
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. QRP aficionados 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. Because of the inherent efficiency of CW signals, a 5-watt CW signal is approximately equal in power to a 100W SSB signal, so in QRP CW is often the preferred mode of operation. PSK31 is also a highly efficient, narrow-band mode that is very suitable to QRP operation.
Equipment
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. Popular US models include Elecraft K2 and K1, the Yaesu FT-817, the Icom IC-703, and the Argonaut V from TenTec Another popular source is Hendricks QRP Kits, which offers a variety of popular kits. HFPack enthusiasts operate QRP radios on the HF bands in portable modes, usually carrying the radios in backpacks, with whip antennas.
Some QRPers prefer to construct their equipment from kits or homebrew it from scratch. The practice of building your own radio is very much alive in the QRP enthusiast community since building a radio for 5W CW can be much smaller and simpler to build than a radio that must do 100W and SSB. QRP'ers are also very likely to be into the practice of just taking your radio out into the field and setting up on a bench or on a mountain top since their radios are often easy to carry and run for quite a long time on battery power. There are even QRP radios that you can build yourself and fit into an Altoids tin.
Contests and awards
There are specific operating awards, contests, clubs, and conventions devoted to QRP enthusiasts.
In the USA, the November Sweepstakes, June and September VHF QSO Parties, January VHF Sweepstakes, and the ARRL International DX Contest, as well as many major international contests have designated special QRP categories. For example, during the annual ARRL's Field Day contest, making a QSO (ham-to-ham contact) using "QRP battery power" is worth five times as many points as a contact made by conventional means.
The QRP ARCI club sponsors 12 contests during the year specifically for QRP operators. QRP-ARCI Contests
Typical awards include the QRP ARCI club's "thousand-miles-per-watt" award, available to anyone presenting evidence of a qualifying contact. QRP ARCI also offers special awards for achieving the ARRL's Worked All States, Worked All Continents, and DX Century Club awards under QRP conditions. Other QRP clubs also offer similar versions of these awards, as well as general QRP operating achievement awards.
"QRP IS FUN", and its a great way to experiment with Ham Radio and a place where you can really build your own radio and have fun using something you have built yourself.
Here are some good links for more information about QRP:
QRP Radios and Radio Kits