The best description comes from the Office of Law Revision Counsel (LRC), here. There are many minutiae of the process that I don't know or understand, so my discussion here will necessarily be an approximation. I welcome any corrections in the comments.
The LRC is charged with codifying Federal statutes, which is essentially organizing them into the Titles of the U.S. Code. However, unless Congress passes a Title as law, and replaces the various laws which make up the Title, the Title will live in a parallel universe from the laws that Congress actually passed. So relying on text in the Title alone can often lead to trouble. The whole Code is revised on a 6-year schedule, so some sections can be as much as 6 years out of date.
The LRC is moving ever faster, and has started to release a "USCPrelim" version, which updates Titles on a faster cycle. However, as long as (1) the Code is not positive law, and (2) changes are not made in a consistent manner, this codification process will continue to require a great deal of manual work and artistry.
At the same time, the LRC has taken up a number of projects to ask Congress to pass certain Titles into "positive law", so that the text of that Title in the Code is the law. There are currently 8 Titles listed on the LRC's website that are, it appears, ready for Congressional action.
Congress could make tremendous progress toward legislative transparency by prioritizing positive law codification, and committing to completion of the process by a certain date (2014?).
Now is a terrific time to start, for a number of reasons:
1. The 6 year cycle completed in January. So the Code is almost completely "up to date".
2. Legislative gridlock on other issues creates a space for passing legislation of this technical nature that has few policy implications, but could offer great gains in efficiency and transparency.
3. Data technologies have advanced to the point that the process of codification can be accelerated. Quality and completeness could be ensured by a number of automated, as well as manual tests. And the benefits of codification would be immediately visible, in the ability to update the Code in real time, just as is currently being done for the Code of Federal Regulations.
This is an exciting time for legislative data, and the House can make changes now, for a relatively modest investment, that will yield benefits for years to come.