Sentry is a Devices Demon Hunter skill unlocked at Level 25. It creates a mechanical turret that fires arrows at enemies, working much like the Wizard's Hydra skill.
The Demon Hunter throws a turret onto the ground that fires arrows at nearby enemies.
<skill class="DemonHunter" rune="Spitfire Turret">Sentry</skill>
<skill class="DemonHunter" rune="Impaling Bolt">Sentry</skill>
<skill class="DemonHunter" rune="Chain of Torment">Sentry</skill>
<skill class="DemonHunter" rune="Polar Station">Sentry</skill>
<skill class="DemonHunter" rune="Guardian Turret">Sentry</skill>
The skill works similarly to those found in the Assassin's set in Diablo II. It is a trap of sorts that fires arrows at enemies. It's not known if the Sentry will go into action as soon as it's cast, or if it only activates when enemies move into range, as the Demon Hunter's other traps do.
It is assumed that much like the Wizard's Hydra, players may only have a set number of traps and/or Sentries active at once.
The following Demon Hunter passives provide a benefit to Sentry, runed or not:
<skill class="DemonHunter">Steady Aim</skill>
<skill class="DemonHunter">Custom Engineering</skill>
Also, if using the Spitfire rune on Sentry, then the Passive skill Ballistics will boost the rockets' damage by 50%
Sentry was first announced in a Blizzard tweet in early January 2011. 
Demon Hunter is getting a few new toys. One of the coolest has to be the Sentry. --Diablo
It was explained in a bit more detail later that month: 
So the Sentry was given to the Demon Hunter. Is this identical to the Assassin’s sentry? I’m dying to know how runes change it. --NocturneGS
The assassin sentries were more like stationary magic dispensers. The demon hunter sentry is more like something a spy would sap. --Diablo
Fuller details came in February 2011, when Jason Bender spoke about Sentry in an interview:
So like I was saying, there are different kinds of traps and they are sort of the usual to some extent and then we have weird some stuff. The Sentry is by default your standard trap as you would expect, it’s a lot like Hydra. I will say that the runes will change how that works.
I don’t want to get into too much specifics about the runes yet, because those are still in development. But you have seen what the runes can do to the Wizard and what kinds of things it can do specifically for the Hydra, but with the gadget theme we can do even crazier and funkier stuff with gadgets, because we’ve learned so much from running for the Wizard’s powers that we can take a lot of that knowledge forward and take some ideas off the shelf and put those into things like the Sentry.
Definitely though, there are a ton of traps that can shoot out ranged stuff that can track you and then there’s traps that cover the ground and if you step on it, it’s going to hurt. So, the vanilla version of the Sentry is just what you expect, it’ll just shoot arrows.
Sentry was first seen at the July 2011 Press Event, where it was a level 22 skill.
When the beta started in September 2011, Sentry was still a level 22 skill, but was soon changed to 21. The skill system was once again overhauled for the Beta Patch 13, and Sentry was classified as an Archery skill (but stayed at level 21). For Beta Patch 14, it was re-classified as a Devices skill and moved to level 25.
Sentry can be seen in action on video here:
You can find pictures in the Diablo III screenshot and picture gallery: