How a can be decoded as 61 and b as 62…? [closed]

I’m trying to stack overflow of one C program. But I’m confused by encoding or decoding algorithms.

I used ollydbg to observe registers. I tried entering "a" 28 times. And it should insert the last "a" in the EIP register because of strcopy cmd used.

When the last "a" is flowed to EIP it reads as 61.

According to ASCII:

A is 65 B is 66 

But the OS reads a as 61.

What is this encoding? I’m very new to encoding.