The trigger beam is simular to the tripwire, however this uses an invisible beam making it VERY hard to detect, it can also go through glass and because it doesn't use string it can't be disabled easily.
It must be made using a Construction table.
The layout to the Construction table doesn't look like that.
The trigger beam can be fitted with a Basic AI that's capable of ignoring specific entities (it's usually un-discriminate, any entity will activate it) this makes it so the user can walk through a base without tripping his own traps.
The trigger beam can also hold a 2nd basic AI, with a 2nd it's capable of making simple decisions based on the the entity that tripped it (and by that I mean, you can set it so if a certain entity trips it a redstone signal is sent one way, and if a different entity trips it a redstone signal is sent in a different direction). This could be useful for making piston-doors that can only be opened by the owner.