I think I'm hearing references to two different things in this thread, and it's important to recognize the differences between each one. In most states, there is a very distinct difference between the "Delinquent Tax Roll" and the "Tax Sale List" - which I think @Jeff Zamora was alluding to above.
Delinquent Tax Roll: This list contains all the properties in a given county that have missed at least one tax payment (as per @Sean OToole's outline), but these properties have not yet been seized by the county in tax foreclosure (i.e. - in California, I believe property taxes can go delinquent for up to 5 years before they're taken by the county).
Tax Sale List: This list contains all the properties that have gone through the entire waiting period and have been seized in tax foreclosure. These properties are now being held by the county, awaiting the next tax sale / auction date, at which point the county will sell these tax deeds (or in some states, a tax lien) to recoup their lost tax revenue.
With the Delinquent Tax Roll (IF you're able to get your hands on the county's list), you can contact the private owners and in many cases, you can structure offers to buy these properties directly from them at a massive discount. This is possible because these individual owners still hold the deed and can control their property.
With a Tax Sale List, the only way to buy these properties is at the county tax sale. There are good deals to be found at the county tax sale too, but at this point in the process, there is a much higher likelihood that you'll have the added burden of more competitors in the market, which could potentially kill any deals you could have gotten ahead of time, if you had just bought these properties directly from their respective owners.
The county's annual tax sale is a very public, commonly known event, and it tends to draw in a lot of eyeballs and competition. As a result, most people are very familiar with the Tax Sale List, because this is what most people are thinking about when they talk about tax lien and tax deed investing.
The county's delinquent tax roll on the other hand, is not a very well known source of information, because most people don't know about it and/or how it can be used to find deals (I've found that most county workers don't even know how to generate it, which is part of why it's challenging to obtain this list in some areas).
All things considered, I've always felt that the Delinquent Tax Roll is definitely the list you want to get. It contains the properties that are still controlled by their respective owners, which allows you to get in and start making offers before the whole world knows about the opportunity. It's a great way to fly "under the radar", because it basically eliminates all of the competitors you would be fighting with at the tax sale. If you can get to these people while they still own their property, you'll have much more time and flexibility in terms of what kinds of offers can be made, and what they'll be willing to accept.