A Lysian sentry pod
Sentry pods were small, unmanned spacecraft used for defense, primarily deployed to intercept and prevent hostile incursions into a certain area.
The Lysians used twenty-nine meter long, unmanned sentries, programmed to defend the Lysian Central Command. The pods were equipped with fusion generated pulse lasers and minimal deflector shielding. The Lysian system had 47 sentry pods around its perimeter. (TNG: "Conundrum")
Three sentry pods of the Mars Defense Perimeter in 2367
The Mars Defense Perimeter, which was part of the Federation planetary defenses, also used sentry pods to protect the inner planets of the Sol system, and specifically Earth, from intruding hostile starships. The defense perimeter consisted of a series of unmanned sentry pods and drones.
In early 2367, an attacking Borg cube crossed the planetary defenses between Jupiter and Mars. It was able to easily break through the defensive lines, destroying the ships sent to intercept it within a matter of seconds. (TNG: "The Best of Both Worlds, Part II")