Action games that focus on the player using some sort of weapon to shoot enemies.
Perspective
Shooters are grouped by the way one sees the gameplay is it has a effect on how the game plays and what actions are possible in the game.
First-person
Third-person.
Top Down
Mechanics
Bullet-time
The player has the ability to slow down time, sometimes so much that one can see individual bullets fly by. The ability is usually limited in some way so the player cant use it all the time.
Cover System
With a cover system the people in combat can take cover behind objects in the game world. A button is often used to move in and out of cover and to move between cover.
Active reload
With this the player has a choice to decrease the load time of a weapon by timing a button press. If the player does not press the button the weapon will reload in normal time. If he miss the timing window the reload will take longer and if he it the window the reload will be faster. By having a even smaller window one can allow for a perfect reload where the performance of the weapon increase until the next reload.
Downed
Instead of dying the player enters a down state where they can not act. They will bleed out and die after a period of time unless they are revived by a teammate. One option is to make the player be able to crawl or fire a sidearm when downed.
Reference
Aiming Mechanics in First Person Shooters
Adventures with Guns in a Third Person Shooter - 2011
Effort Upon Effort:japanese Influences In Western First - Person Shooters - 2015