ARM vs Intel

The Central Processing Unit (CPU) is the “brains” of your device. Its job is to execute a sequence of instructions to control the hardware on your device.
The speed and power efficiency of these CPUs is critical. The speed affects the user experience, while the efficiency affects the battery life.
 
At the highest level, the first difference between an ARM CPU and an Intel CPU is that the former is RISC (Reduced Instruction Set Computing) and the latter is CISC (Complex Instruction Set Computing) 

RISC instructions sets are smaller, more atomic, while CISC instruction sets are larger, more complex.
By atomic, I mean that each instruction roughly translates to a single operation that the CPU can perform, e.g. add the contents of two registers together. CISC instructions express a single idea, but the CPU will need to execute 3 or 4 more simplified instructions to perform it. For example a CISC CPU can be told to add together two numbers stored in main memory. To do this, the CPU needs to fetch the number from address-1 (one operation), fetch the number from address-2 (second operation), add the two numbers (third operation) and so on.
  
The next major difference between an ARM processor and an Intel processor is that ARM has only ever designed power efficient processors. Its raison d’être is to design low-power usage processors. That is its expertise. However Intel’s expertise is to design super high performance desktop and server processors.  
The average Intel i7 processor produces around 45W of heat. The average ARM based smartphone SoC (including the GPU) has a maximum instantaneous peak power of around 3W.