I'll do my best to explain. I need to start by noting that, as far as I'm aware, there's no generally accepted nomenclature for this stuff. Accordingly, I've invented my own terms (some of them as I write this).

At a high level there are two different kinds of brokers, which I'll call "primary brokers" and "secondary brokers." Primary brokers are companies that primarily aggregate their own data and sell it to other businesses. Secondary brokers are companies that primarily buy data from primary brokers and resell them to other companies or to consumers.

Each broker can further be defined by the kind(s) of data they sell. Generally speaking, there are 5 categories of data:

1. FCRA records. These are mainly credit records and credit-related data that are permitted to be used in making credit determinations.

2. Non-FCRA public records. This is stuff like birth, death, marriage, property records, DMV records, criminal records, etc. These are explicitly not to be used for credit and hiring determinations (not sure about renting, off-hand).

3. Hiring data. Frankly I'm not entirely sure what these are beyond criminal background stuff, but they are allowed to be used for making hiring decisions, where permitted.

4. Marketing/Lifestyle data. This is stuff like purchase history, magazine subscriptions, hobbies and interests, data sold by companies whose products you sign up for for free, etc.

5. Insurance underwriting data. These tend not to be explicitly tied to a non-anonymous person. However, given someone's street address, you'd be able to pull some info on their property or something.

Most brokers sell data belonging to multiple of the above categories.

The big primary brokers for FCRA data are, as you might expect, credit agencies (Experian, Equifax, and TransUnion).

The big brokers who deal in mixes of data are LexisNexis (a Reed Elsevier company), Thomson Reuters/WestLaw, and TLO (a TransUnion subsidiary). They are primary brokers of Non-FCRA and hiring data but secondary brokers of FCRA data (I believe they mainly purchase it from the credit agencies). There are a couple small companies that are primary brokers of things like hiring data (e.g. Checkr).

The big primary brokers of marketing/lifestyle data are companies like Acxiom, BlueKai, Epsilon, US Data, and a number of others. I have heard that credit card companies sell certain kinds of data but I am not sure what exactly and have not used their products myself.

The big primary broker of insurance underwriting data (along with a couple other categories) is CoreLogic.

The big secondary brokers of Non-FCRA data are Intelius and Instant Checkmate. I do not think they are primary brokers under my definition because most of what they sell is basically an identical copy of what the primary brokers sell, but for a lot more money. It's hard for me to imagine them taking on the expense of aggregating the bulk of their own primary data when they can run a search against a primary broker DB for a few bucks and resell a pretty version of the response to a consumer for $50. They probably aggregate some of their own data but I'd guess it's low-hanging fruit.

There are also some purely social media-driven secondary brokers, like Pipl.