Even though the number has steadily decreased since the 90s, there are still many different and incompatible CPU architectures in use nowadays. Most computers use x86_64 and pretty much all mobile devices and recent Macs use some kind of ARM64-based ISA (instruction set architecture).
In specific fields, though, there are more exotic ones: most routers still use MIPS (for historical reasons), a roomful of developers use RISC-V, the PS3 used PowerPC, some servers 20 years ago used Itanium, and of course IBM still sells their S/390-based mainframes (now rebranded as z/Architecture). The embedded world has even more: AVR (used in Arduino), SuperH (Saturn, Dreamcast, Casio 9860 calculators), and the venerable 8051, an Intel chip from 1980 which is still being produced, sold and even extended by third parties.
All these architectures differ on their defining characteristics, the main ones being:
- word size: 8, 16, 31, 32, 64 bits, sometimes more
- design style: RISC (few instructions, simple operations), CISC (many instructions, performing complex operations, VLIW (long instructions, doing many things at once in parallel)
- memory architecture: Harvard (separate