The Sentinels were a type of robot warriors used by the Decepticons during their control over Cybertron.
They appeared as bipedal domed headed machines capable of speech and made use of energy pistols. They possessed a chest screen that would open up allowing them to report to their commander. Unlike their masters, they did not possess the ability to transform. They were also in charge of leading Guard Decepticons.
The true nature, ranking and capabilities of the Sentinels were never explored and its entirely possible that they are simple programmed machines rather then a sentient construct like the Transformers.
Fiction
Generation 1 cartoon
After Spike Witwicky along with Carly arrived on Cybertron and managed to escape Shockwave, the guardian of Cybertron dispatched a Sentinel to deal with the Humans. The Sentinel would locate the pair along with the Dinobot Swoop and stunned the Transformer taking them to the Cybertonium Pit. After a fight broke out between Grimlock and Spike within the Pit, the Sentinel requested instructions from Shockwave on what to do and was told to remove the Humans. After lowering the energy, the Dinobots attacked and the Sentinel was melted by Slag's fiery attack. DotD2
The next point the Sentinels were encountered was when Shockwave deployed them in military manevuers and used holograms for target practice. A sizeable army of Sentinels were present when an asteroid arrived which carried the banished Combaticons. The Sentinels were forced to defend Cybertron from this threat which combined into Bruticus. The Sentinels were decimated with many destroyed and others confined within the detention area until freed by a Megatron's Decepticons who were briefly allied with Optimus Prime and the Autobots who worked together to defeat the giant Bruticus. TRoB