Ranged Combat and Shoot Attacks are closely related terms, but the Chronicle X rulebook tends to use Ranged Combat to refer to the general concept of attacks at a distance, while Shoot Attacks refer to non-Melee attack actions.
Shoot
Shoot Attack refers to the attacks at a non-Adjacent target. Each Model and Alien Unit potentially has two Shoot Attacks available: short range and long range. They are listed on the stat card, usually with the short range attack's range and Shoot Attack Value on one line, and then the long range attack's range and Shoot Attack Value on the next line below the short range attack. A Shoot Attack is a short range attack if it is within the short range limit, and otherwise it's a long range attack if its within the long range limit. (You can't choose to use a long range attack if the target is within short range.)
Once the combat type and relevant Combat Values have been determined, all Attacks are handled in the same basic way.