Currently there are 3 main CPU architectures on the market, some of them could be divided in certain categories (not taking into account extensions like MMX or Velocity Engine - AltiVec), some even overlap.
Classic CISC (Complex Instruction Set Computing) CPUs are history on the modern PC arena. However, they are still widely used in various electronic devices (for instance RAID controllers). Intel 486, Pentium (not Pentium II), Motorola 68k series are examples of classic CISC. Classic CISC CPUs have very complex instruction set, instructions may have different length. Complex CISC instructions may need tens of CPU clock cycles to execute. CISC CPUs hit the wall at certain moment because logic becomes too complex for further performance improvement.
RISC (Reduced Instruction Set Computing) designed to simplify CPU logic. Close examination of CISC CPUs revealed that 20% of instructions executed 80% of time. So, it was obvious to eliminate rarely used instructions and implement them in software. RISC instructions have fixed length and in most cases need only one CPU clock cycle to be executed.
Intel and AMD, two bigger players on PC arena, and newcomer NexGen found smart way to upgrade outdated architecture (later AMD bought NexGen). The idea behind Intel Pentium II/III, AMD K6/K7 is simple - RISC core plus special decoder which translates CISC instructions into RISC ones. Internal RISC instructions are not available for external access, however. Of course, decoder adds several millions of transistors, but for the modern CPU that's almost nothing (taking into that the fact that even some Video Card Processors have 30+ millions of transistors). These CPUs retain full compatibility with old software and offer speed which in many cases match or even exceed its RISC counterparts.
VLIW (Very Long Instruction Word) have a long history with a lot of casualties (in fact, all VLIW pioneers bankrupted). The idea behind VLIW is to combine several RISC instructions that could be executed in parallel into one big instruction. Itanium (Merced), jointly developed by Intel and HP, and Transmeta Crusoe are the first VLIW CPUs materialized for PC (namely as PC's CPU, because some multimedia processors are based on VLIW). Intel promotes its variation of VLIW as EPIC (Explicitly Parallel Instruction Computing), probably because of negative aura around VLIW and to distinguish itself as inventor of radically new CPU architecture. Another CPU architecture based on VLIW is SUN MAJC (Microprocessor Architecture for Java Computing).
VLIW Code Morphing
What is real advantage of VLIW CPUs? Continued - VLIW CPUs...