Killing machine designed for one thing."”
"Search and destroy.
― Trinity and Dozer[src]
A sentinel, also referred to as a "squiddy" by the Human Resistance, is an autonomous machine that serves the Matrix in reality. In series they are typically seen patrolling the ancient sewers and passageways of the dead human cities of old in search of human-controlled hovercraft or wandering humans, but the lack of dedicated military weapons could also mean that they spend most of the time maintaining the Matrix hardware.
The sentinels replaced the B1 series after their exile by the humans. Over time, the machine AI grew more intelligent, and they built newer, better upgrades for their appearance.
Sentinels were originally built as construction unit and given military tasks much later. Since they lack dedicated offensive weaponry, their fangs and welding laser beam give away that they are much more fit for construction tasks, which was their primary goal - maintenance and repair of the real-world physical Matrix structures. When the Machine War broke out, the sentinels were deployed as a search-and-destroy unit, and proved to be more than a match for the war machines used by the United Nations.
They were tasked with picking up the dead and incapacitating live humans in order to study them. In addition, the Machines initially relied on sentinels to maintain their power plants before eventually replacing the sentinels in that capacity with specialized medical hoverbots. After the conclusion of the war in the machines' favor, the sentinel units were further upgraded, and now patrol the sewers of the dead human cities of old, where they search for wandering Zion vessels or humans unlucky enough to be walking through them at the time.
A sentinel appears as a multi-tentacled creature with multiple sensors that moves effortlessly through the air, probably using an advanced internal form of pad technology; one can easily catch up to a fully-powered human hovercraft. A sentinel's frightening appearance reminds many humans of squids, where they get their nickname of "squiddies". They tend to fly in swarms like squids, that can range from a few to thousands.
In addition to their powerful, clawed tentacles, sentinels are armed with a continuously firing laser beam that can slice and weld metal structures with ease, including a ship's hull. They are equipped with multiple audio and visual sensors, which look like satellite dishes and eyes. Sentinels are capable of electrical signature detection, such as generated by a fully-powered hovercraft, and will relentlessly follow that signal to its end.
Sentinels are also armed with tow bombs, tiny machines that seem like a sentinel's larvae, but are actually guided explosives that will tear through and destroy a hovercraft with ease. A sentinel will deploy such a bomb by grasping it in a single tentacle, spinning very fast and releasing it with great velocity toward its target.
The sentinel's multiple appendages can be used as an ablative armor, deflecting projectiles like bullets from the vital "heads" (where programming data is stored) of close-by sentinel allies, explaining the swarm tactic.
There is no negotiating or reasoning with a sentinel. They only receive orders either from within the Matrix by Agents, or by the Deus Ex Machina in the Real World, and will pursue the completion of their mission relentlessly unless destroyed or until the order is rescinded.
While hovercraft are normally needed to stop an attacking sentinel, a lucky human armed with a lightning rifle may be able to destroy one. A human in command of an APU can also deal sufficiently quick damage to approaching sentinels, but are at the sentinel's mercy if their APU is within close range, as the APU can be pushed over or the sentinel's ripping claws can tear the relatively unprotected human controlling the APU.
In a mysterious display of his powers outside the Matrix, Neo is able to command Sentinels and bombs to deactivate or detonate with only the powers of his mind.