Different Android devices use different CPUs, which in turn support different instruction sets. Each combination of CPU and instruction set has its own Application Binary Interface (ABI). An ABI includes the following information:

The CPU instruction set (and extensions) that can be used.
The endianness of memory stores and loads at runtime. Android is always little-endian.
Conventions for passing data between applications and the system, including alignment constraints, and how the system uses the stack and registers when it calls functions.
The format of executable binaries, such as programs and shared libraries, and the types of content they support. Android always uses ELF.