For example, if you just want user mode cpu emulation for X86-64 architecture, you need to use qemu-x86_64 binary and if you need whole X86-64 bit system emulation (like your PC), qemu-system-x86_64 binary must be used in commands below. Newer distributions have separate binaries for these two different operation modes. If you want to simulate whole system not just the cpu (like a PC) you need to use full system emulation mode. Qemu has two operating mode named full system emulation and user mode emulation.
