The basic function of an A&R person is to find amazing talent for the label he/she works for; talent that hopefully will sell lots of records/have a long career. Hence, the A&R function exists at labels both large and small. Major labels might have designated A&R reps, while an A&R person at an indie might also perform other functions (e.g., sales, marketing, janitor). In general, once the A&R person signs an act, he/she would be involved in helping administer the making of the record (i.e., setting budgets, selecting producer, choosing songs, etc.), and ultimately helping create the marketing campaign/imaging the artist. The A&R person is normally the biggest champion at the label for the artist, and it is wise for the artist to keep close communication with that person. Unfortunately, there is a lot of turnover at the A&R level and some new A&R types might make an unwise decision that damages an act, so an artist would be well advised to get everyone at the label behind the project to ensure it has the best chance of success.
Ben McLane, Esq