A junior sysadmin executes checklists, writes simple scripts, reads logs, and solves common problems, all while learning the complex supersystem of their workplace. They learn configuration of systems in use and how infrastructure is arranged and commonly changed.

A sysadmin helps to create checklists, writes more complex code to work in an automation system, and generally solves problems with non-obvious causes and/or remedies. They know all the routine events in their workplace, and add to documentation. They build, configure, test and recommend new tools and solutions to users, devs and the rest of the ops staff.

A senior sysadmin works on policy, debugs automation, solves complex interactions, and has an excellent understanding of complex systems and networks, not just at their workplace but in interactions and hypothetical interactions with the Internet at large.

DevOps guys are senior sys admin in the cloud (and that last caveat isn't mandatory). People shit on sys admins all the time but the only real difference between senior sys admins and DevOps engineers is pay packet, tools and responsibility.