Legislative bodies all across the United States and at all levels of government, from city councils to the U.S. Congress, are busy all the time passing new laws. Many shelves of bookcase space is devoted to just 200 plus years of Federal legislative enactments. Keeping track of it all and making it as accessible as possible is a big job.
A single Act of Congress may have many different part, on several different subjects and it may reference or impact upon numerous preexisting laws.
The Dodd–Frank Wall Street Reform and Consumer Protection Act of 2010 is a good example. Dodd–Frank addresses both a reform of Federal securities law and a reform of Federal consumer protection laws. The first focuses upon the activity of financial institutions, which is mainly of interest to Wall Street lawyers, and the second focuses upon such matters as consumer credit cards and payday loan, which is of interest to everyone else.
The Congressional Act itself, with all its various parts, is officially known as Public Law (or P.L.) 111-203, 124 Stat. 1376, and you can read the Public Law version online courtesy of the Government Printing Office. As an aside, the '111' of P.L. 111-203 signifies the 111th U.S. Congress (2009 - 2011).
The various Public Laws and other Acts of Congress are collected and published at the end of a Congressional session in the U.S. Statutes at Large (Stat.) The United States Statutes at Large are the official source for the laws and concurrent resolutions passed by the United States Congress.
The United States Code (U.S.C.) is created and maintained by a group of busy professionals who take the different parts of Congressional Acts, divide them up into logical groups and arrange them by topic. This way, all the Federal laws on Consumer Protection, for example, can be conveniently found all in one book.
This is codification, which is the process of creating a code. The U.S. has the United States Code, every State has its own state law code, and local governments have their versions too, all arranged by subject. Think building code, zoning code, fire code etc.
For the vast majority of day-to-day legal research and writing, it is to the various codes that we turn and to which we make reference.
United States Law Online: United States Code, Statutes at Large, and Public Laws, The Law Library of Congress.