During his mission, Michael Thorton can come across documentation and records regarding other characters and factions, friend or foe alike. Information regarding people or factions are compiled as dossiers. There is also available intel on his current mission: security details or surveillance and mapping of where he's going. This intelligence can be bought off of The Clearinghouse for various sums of money.
Intel can be obtained by hacking into specific computer terminals, in which case Mike will email the files to himself.
Dossiers provide insight into the story and the backgrounds of each character or group. Dossiers are useful gameplay-wise because they contain details on combat tactics or habits of the subject, useful for missions.
When intel is bought, Mike posts on The Clearinghouse what he's looking for, somebody will email him and send him the details he needs.