[Editor's note: If you're reading the Building a Voter File series in order, you should be aware that this particular entry is not part of the main sequence; thus, Errata.  I will return to the tutorial in short order.]

As Dirty D points out, I can indeed lay out some cautionary tales regarding the construction of vote history.  This can be an extremely important field, too, so if you're using vote history it behooves you to be an active consumer.  Ask questions of your vendor, and don't be shy about digging into the data.

Once you've gone through this process, you should have a list with millions of entries, each containing personal and consumer information--ideally for every registered voter, and all non-registered adults.  So what can you do with it? Plenty.

Once it's compiled, the data has to be accessed.  Various people can be granted different levels of access--making the whole file available to any volunteer would raise serious privacy concerns, not to mention possibly giving access to rival campaigns or, god forbid, the other party.  For low-level volunteers, this access can be extremely limited, while higher-level operatives can be granted more generous permissions.  Broader access can be granted through a web interface like the DNC's Votebuilder, RNC's Voter Vault, or Catalist's Q-tool.  Using some relatively simple Boolean logic, you can create lists of all the people in a state, district or precinct who share certain characteristics--for example, you might want to find all registered black voters under the age of 40.  With a certain (ever-diminishing) amount of inaccuracy, this is a trivial list to pull.

As you can imagine, this is extremely useful.  You can use these tools to do everything from create walk lists for your volunteers to pull samples for polls or blanket a state with direct mail.  Which is why these files are considered so valuable, and why making them is big business--with big consequences.