Purchase collections from various dealers to showcase.
Coordinate with the owners of said collections that are to be shown on display at the museum.
Coordinate with the marketing, sales people to ensure that new collections increase museum revenue, public relations, etc.
Ensure that the overall theme, goals of the museum are reached through maintenance of current collections (eg, cleaning, upkeep, etc) as well as new ones.
Work with art dealers, auctions, etc, to potentially sell and or buy various related works for the museum.
Work with other museums to trade collections, to increase revenue, sales, etc. Eg, more new exhibits, more regularly, means more sales, but you'd have to manage the trade off of the cost of new exhibits vs the incremental revenue they bring in.
That's my guess, anyway, if you interview one, I'm sure they have more detailed and or specific information.
Ok, thanks a lot! I just needed the general idea, and since I'm not natively english I had no idea what a collections manager was. :)