What is the difference between SRE and DevOps?
DevOps is about maintaining code, is that correct?


DevOps is not "just" about code, or systems, or any one thing.  DevOps is a very general term that covers all things related to software delivery.

Site Reliability Engineering is a term popularized by Google. From this article https://landing.google.com/sre/interview/ben-treynor.html we can distill their TL;DR:


  Fundamentally, it’s what happens when you ask a software engineer to
  design an operations function.


Operations, Engineering, and Software Development are blurring together.  The degree of automation required to create and maintain a mature infrastructure requires skills from all three.  SREs are admins, and engineers, and developers.

See also: 
http://shop.oreilly.com/product/0636920041528.do