What is the difference beween snapdragon 820 and 625

They are totally different and 820 beats it in every department.

Main Difference is as follows:

1. Core- snapdragon has four custom cryo cores whereas snapdragon 625 uses eight Arm stock A53 cores. Although 625 has more cores but All cores are A53 cores which are energy-efficient cores (geared towards energy efficiency rather than performance, don’t have ooo (out-of-order execution etc.) Whereas 820 is built on qualcomm custom ‘kryo’ core which is high-performance core (geared toward performance rather than energy efficiency and have out-of-order execution).

2. Processor clock speed- snapdragon 820 is divided into two cluster (each of its cluster has two core) where one cluster can go to 2.15 ghz the other cluster can go to 1.6 only. Whereas 625 can go upto 2.0 ghz for all of it’s eight core.

3. Gpu, it’s clock speed and API support- snapdragon 820 has adreno 530can go up to 624 mhz and supports following api

• OpenGL ES 3.2
• OpenCL 2.0 full
• Vulkan 1.0
• DX11.2

while 625 has adreno 506 gpu which can go upto 650mhz but has less cores than adreno 530 and supports:

• OpenGL ES 3.1+

4. Cache- snapdragon 820 has larger L1 and L2 cache.

These are the main difference, there are others like in dsp, isp, modem etc.
But 820 excels in all departments and wins hands down!