What are QRP, QRPp, QRPpp and Milliwatting in Amateur Radio?

What are QRP, QRPp, QRPpp and Milliwatting in Amateur Radio?

QRP means operating at low power, typically 5 Watts or less for CW or 10 W PEP or less for SSB. There are many contests and awards with special sections for QRP. QRP operation, especially on battery power is often a multiplier in contests. A good antenna set-up and feeder cable will boost your QRP performance. A lot of patience and good knowledge of band conditions and operating procedure of DX stations is needed if your planning QRP DXing.

Now what is milliwatting? It is operating with power less than a Watt. Some classify QRPp as power between 100 mW and 1 W and QRPpp as power below 100 mW. I was wondering what one can do with power less than 100 mW till I read the QRZ.COM page of one Qatar Oscar 100 Geostationary Satellite operator. The first contact on QO 100 for that person was on 70 mW. But that is not on HF, but at 2.4 GHz QO 100 uplink and satellite downlink at 10 GHz.