Credits are the currency of the game. Usd for buying Weapons, gear and mods from the Vendors. There are 3 types of credits.
The main credits earned from doing missions in the city can be used at the main Vendors in the cities safe houses.
Dark Zone credits are earned by killing enemies in the dark zone and collected from where they fall. These are only used in the dark zone Vendors. Remember that you will lose some credits each time you die in the dark zone so your total may go down as well as up.
Phoenix credits. They can be spent in the special gear vendor in the Base of Operations or the special Dark Zone vendor found in the last (highest) dark zone area DZ06.
These can be earned after hitting the level cap at 30, by replaying the missions at Challenge difficulty, completing daily challenges and killing special names enemies throughout the city.
Target intel
Target intel are credits you receive from the Search and Destroy Assignments added in Update 1.2. They are then needed to purchase / activate the High Value Target missions available from the HVT officers in the Base of Operations.