There's an interesting article over at LineProducing.com, though I'm not sure where, as I found this posted on one of the many message boards I'm a member of. (In fact, I can't believe there actually is a LineProducing.com.)
Anyway, with so much confusion in Hollywood over what a producer actually does, I thought this might help explain what this particular type of producer, a line producer, does.
In a nutshell, Line Producers create a budget (usually several budgets) for a production - then are responsible for keeping to the budget during production. Line Producers are 'on the line' or 'responsible for each line' of the budget. They are involved with all aspects of the production and are directly in contact with the rest of the crew and the producers above. Line Producers typically hire the key crew (along side the director and producers) like the Director of Photography and Production Designer. Line Producers oversee the hiring and approve everyone else the UPM would like to bring onto the project.
A UPM, Unit Production Manager, on the other hand is the person who executes the plan and reports to the Line Producer. They are the person dealing with the daily approval of timecards, reviewing the production reports and approving call sheets. The Producer's Guild of America (PGA) counts many Line Producers among it's membership (but does not officially recognize them) - while the Director's Guild of America (DGA) officially covers the union status of UPM's.
It can be argued that a Line Producer is a non-union UPM, and vice-versa, but there are many shows which have both positions. However, these tend to be larger budget project. There is a fine line of grey which differentiates the two positions. Sometimes they are the same person.
For further clarification, the differences between a Unit Production Manager (UPM) and a Production Manager is nothing - except a little knowledge of Hollywood history. Back when studios had in house production departments, there was a production manager (today they are called production executives) and each project was completed by a 'unit' of crew and cast. Remember, the old studio system contracted talent (cast and crew) to work exclusively for the studio. These 'units' had their own Production Manager. As things evolved, these UPMs became freelance, but never lost the 'Unit' part of their title.