The Office of Management and Budget is part of the executive
office of the president. It reviews and approves proposed federal
agency budgets before forwarding the budget request to Congress.
Only Congress can appropriate money for federal agencies, although
as a rule of thumb, they end up modifiying only about 10 to 15
percent of the budget request.
The timeline goes something like this: since the federal fiscal
year starts Oct. 1, the president has to send the budget to
Congress by early February. So, OMB reviews agency spending plans
during the summer, sending a "passback" (i.e., proposed changes) to
agencies in the fall. A final consolidated budget is then sent to
Congress by late winter.
The budget side is the largest part of OMB. The "management"
side issues directives for federal agencies to comply with in the
managament of information technology projects, financial
statements, etc. How seriously those directives are taken depends
partly on who issues them and the size of the receiving department.
A smaller agency has more to fear from OMB then, say, the Defense
Department.