A Point Defense, commonly abbreviated PD, is a static defense against attacking land and sea surface units that are within range of land. It has higher hit points than equivalent mobile units, and it is more deadly. A point defense can attack neither submerged enemies nor air units. It is very vulnerable to bombers and gunships, as well as artillery and missile launchers that have greater range than the point defense.
A single point defense is rarely effective. They are much more effective in groups that have overlapping ranges. They should not be built right next to each other because of the area of effect of enemies' weapons - particularly T1 PDs, as their main enemy, T1 mobile light artillery has a blast radius. Point defenses are on tall towers, so they can easily shoot over nearby walls. Those same walls are often high enough to block incoming fire from enemy units, especially Tech 1 non-artillery units, so walls make point defenses significantly hardier.
All factions benefit from using both T1 and T2 PDs together (for different reasons). For Cybran, UEF and Seraphim, using T1 PDs in addition to T2 PDs brings up DPS, whereas for Aeon it allows for lower overkill, taking care of T1 armies (and other units with low health) much more effectively. Even the UEF Ravager should be built with T2 PDs assisting it, due to its 8 second reload time. As Ravagers also require energy to operate (unlike any other PD), it's also worth building others around it, so that if you lose power briefly, your defensive line will not be rendered useless.