什么是交叉编译器和跨平台？

I am bit confused with the terms of cross platform and cross compiler. Let me be first clear about the cross platform. When people talk about cross platform it means the architecture of the CPU, i.e, X86 and X64 ? or the OS i.e windows 32 bit or Windows 64 (server or desktop)? UNIX doesn't have different flavours like 64 bit or 32 bit. Why so? I am bit confused with the terms of cross platf