Hey GNS,
There are two diffrent types of triggers: analog and digital. Digital triggers use TTL/CMOS logic and can be set to trigger off rising or falling edges on PFIn, RTSIn, and GPCTR0. The analog triggers only work for specific devices and can trigger off of a level on any channel or PFI0. Refer to the help on AI Start.VI for more information. There are several analog and digital trigger shipping examples that install with LV. Take a look at them.
I hope this helps out.
Joshua