The Sentinels were large robots that killed or rounded up mutants and other beings with superhuman abilities. A single Sentinel was referred to as a "unit." Jean-Luc Picard observed that the Sentinels, despite being destructive, were "slow and clumsy." They were active in a post-atomic New York City in approximately 2013 of a parallel reality.
When a team made up of X-Men and USS Enterprise-E crew were sent to this point in history by Kang, three Sentinels detected the use of mutant abilities and searched out the team in order to destroy the mutants. When the Enterprise crewmembers posed a threat, they also became targets.
Deanna Troi damaged one Sentinel with a handheld phaser, leading the Sentinel to determine Troi as an unknown non-human, needing to be vivisected and studied. Picard destroyed it with a phaser blast while the X-Men destroyed the other two Sentinels. Eventually, two hybrids (a mix of Borg and Sentinel) tracked down the team after meeting with resistance member Tasha Yar. (TNG - X-Men comic: "Second Contact")