The basic difference is how long the fluorophores will fluorescence after excitation.
Both fluorescent and time-resolved fluorescent molecules need to be excited at specific wavelengths of light. After excitation, energy is released as light from these molecules. The emitted light for FI molecules occurs in nanoseconds after excitation, whereas emitted light from TRF molecules occurs in microseconds after excitation.
Greater sensitivity is able to be achieved in TRF assays because the longer fluorescent lifetimes allow for a longer delay in between the background excitation light and the measured light emitted from the assay. Having a lower background thereby gives better signal to noise ratios, which in turn increases sensitivity.
GeneBLAzer® is an example of FI assay known as FRET.
HTRF® and LanthaScreen® are two different types of TRF assays known as TR-FRET.
For more information on Fluorescence Intensity and Time-Resolved Fluorescence as well as videos and microplate readers that can perform these measurement modes, visit http://www.bmglabtech.com/technology/detection-technologies.cfm