Microchip Technology does not use PIC as an acronym (in fact they call their chips PICmicro's). It could be regarded as an acronym for Programmable Intelligent Computer, the General Instrument name for the PIC1650.
The original PIC was built to be used with GI's new 16-bit CPU, the CP1600. While generally a good CPU, the CP1600 had poor I/O performance, and the 8-bit PIC was developed in 1975 to improve performance of the overall system by offloading I/O tasks from the CPU. The PIC used a simple microcode stored in ROM to perform its tasks, and although the term wasn't used at the time, it is a RISC design that runs one instruction per cycle.
In 1985 GI spun off their microelectronics division, and the new ownership cancelled almost everything — which by this time was mostly out-of-date. The PIC, however, was upgraded with EPROM to produce a programmable channel controller, and today a huge variety of PICs are available with various on-board options and program memory from 512 instructions to 8k and more.
The bit size of PICs is a source of much confusion. All PICs handle data in 8-bit chunks, so they should be called 8-bit microcontrollers. But unlike most CPUs PICs use a Harvard architecture, so the size of an instruction can be different. In fact different PIC families uses different instruction sizes, which makes it a challenge to compare the code size of PICs to other microcontrollers.
The old PROM and EPROM PICs are now gradually replaced by chips with Flash memory. Likewise the original 12-bit instruction set of the PIC1650 and its direct decendants is superseded by 14-bit and 16-bit instruction sets.
PICs are well-covered on the internet, mainly for two reasons. First some popular consumer products (pay-TV, playstation) used PICs as part of their security system, which attracted the attention of crackers. Second the (now obsolete) PIC16C84 was the first widely available microcontroller that could easily be re-programmed by hobbyists.
Every now and then there are companies that offer cheap and/or enhanced PIC versions. Most seem to disappear quite soon. Scenix (formerly Ubicom) seems to avoid this fate and produces PIC clones that run much faster than the originals.
The rfPIC microcontroller devices integrate the power of Microchip´s PICmicro devices with UHF wireless communication capabilities for low power RF applications. The devices offer small package outline and low external component count to fit the most space-constrained applications.
Microchip (http://www.microchip.com) Ubicom (http://www.ubicom.com) PIClist (PIC mailing list) (http://www.piclist.com) page with links to the original PIC1650 documents (http://www.idcomm.com/personal/ottosen)