For example: I live in the 9th Congressional District of Massachusetts. The boundaries of my district are pretty straightforward, except in the case of the city of Fall River, where the boundary line seems to meander arbitrarily along different streets, dividing the city between the 9th and 4th districts. Surely there must be some kind of formal definition of where the boundary lies, so that voters know who they are eligible to vote for and where to find their polling place. Does there exist some kind of description from the government on precisely where and how the lines are drawn? I couldn’t find anything from the U.S. Congress or Census websites.
Definitions: Your State's Laws
Congressional boundaries are set by states, not the federal government. According to the Massachusetts Secretary of State you can find the current definitions here (2011 Session Laws, Chapter 152, Section 3).
An example definition from Massachusetts is:
Bristol and Norfolk - Consisting of the city of Attleboro, ward 3, precinct B, ward 4, precincts A and B, ward 5, precincts A and B, ward 6, precincts A and B, and the towns of Mansfield, Norton, Rehoboth and Seekonk in the county of Bristol; and the towns of Foxborough, Medfield, Sharon, precincts 1, 4 and 5, and Walpole in the county of Norfolk.
The Census Bureau: Maps
You can download the shapefiles here, which are viewable in a GIS application.
You can also get PDFs of each individual state's congressional bounderies (or purchase a poster) here.