This is one of those questions we get every so often at the Reference Desk, so I thought it might be interesting to share some links that answer this question on the blog.
The Constitution provides for Congress to appropriate money to be spent by
the Federal Government, so when the White House issues the Budget of the United
States in February of each year, it can be confusing.
LLRX.com has a detailed
explanation of the legislature's role in the federal budget process in CongressLine: The
Budget. They also have a great discussion of the role of Congress as the appropriator of monies in CongressLine:
Authorization and Appropriation. A discussion of the earmark, legislative language specifying how funds in a specific account are to be spent, can also be found in a 2 part series on LLRX.com.