As other platforms are already explained (i.e. X86 for 32 bit, x64 is for 64bit only, and ‘Any CPU’ can run in Both) I’ll concentrate on Mixed Platform and how this is different from Any CPU.
The Any CPU is at the project level setting, whereas in the real world solution we have a number of projects under one solution, and there’s a chance that some of my project use Any CPU but other uses the x86 or x64 build platform.
So at solution level automatically Mixed Platform will be selected. This indicates that during Build/Rebuild solution each project builds based on its selected platform.