In generally, it is collection of financial resources.
Why is fund needed ?
You would see the fundraising department in NGO, the volunteers of NGO create fund and collect money for social works. But in corporate sector, fund is collected for development of company and earning on investment or repayment of loan.
Learn Fund in Plain English from this Video
Accountability for usage of fund
If fund is created from public money, then trustee of fund are accountable about its usage.Fed Vice Chair Donald Kohn testified before the Financial Services Committee, along with John Bovenzi of the FDIC. The Fed's balance sheet has expanded by $1.2 Trillion Slush Fund. since September 1. Where did the money go? Kohn said usage of this fund. But I think that he did not explain clearly.