There is actually no set definition to what a Stage Manager's job amounts to. Or rather, it differs fairly widely from theatre to theatre.
One of the largest qualifiers in how many duties a Stage Manager will have proceeds directly out of how large the theatre company is, and thus how much can be delegated.
Generally, the Stage Manager's job will always require him/her to meet periodically with the Director to check up on details, requirements, etc. The SM checks up on production, resource requirements, etc. This may even require that the SM order materials, arrange for their pickup, and look out for some measure of accounting.
The SM is also responsible for setting back stage order. What is the most efficient way to run the backstage, where should the props be placed and in what order, who needs to be in charge of food preparation, consumables, etc.? This also includes guarding sight lines, keeping order back stage, making sure that the actors are on stage at the right time (if they can't be bothered to manage themselves), and all other jobs which require order.
The SM generally has no design responsibilities. They generally aren't involved in direction, acting, or any other stage involvement. Their sole involvement is logistical.
However, it generally behooves the Stage Manager to be aware of situations occuring between actors and stage crew, and being able to encourage and/or mediate between the cast and crew can go a long distance toward preserving the longevity of the production.
All in all, the SM job is much like being a parent in a large and hastily organized household. | 
