What is the difference between Sysadmin and DevOps Engineer?
Mainly DevOps is not a role (when used as such it's more a buzzword than a real role).

DevOps is roughly an organization pattern aiming at breaking the silo between developers and sysadmins.
The main goal is to build teams with devs and sysadmins (along with testers usually) responsible for a product (application) from its definition, architecture decisions up to the maintenance in run of this product.
Each member of the team will be part of the decision on the whole life-cycle of the product, a dev will do some sysadmin tasks in production, and a sysadmin will participate  in the design phase of the product to avoid caveats from the infrastructure perspective for example.

At ideal, a sysadmin would also be part of the development team for the product, in real world sysadmin code more on the configuration around the product and monitoring solutions, but being able to voice concerns to other members of the team avoid a lot of misunderstanding on the deployment process.