Killstreaks are powerful abilities of each hero that can be activated after scoring certain amount of kills. They are the strongest powers anybody can have, but they are temporary and last shortly, unless they are killstreak weapons. Killstreaks depend on classes.
While killstreaks do not have restrictions, they can be offensive, defensive and support.
Turrets are used by Engineer. They are defensive, support and offensive at the same time, as you can place them to defend points, drop them in enemy crowds to save ammo, or support your team anywhere on the battlefield.
They are used only by the Juggernaut. Both defensive killstreaks give him an armor set, along with shield that has special effects.
Defensive support killstreaks are used by Medic. They heal the squad, or boost their defense.
List of killstreaks
Killstreak weapons are a special type of Primary Weapons that act as weapon when they are activated, but they also appear in custom forms in side gun games.