18X Optical Zoom refers to the actual lens magnification. This magnification is true magnification at 100% resolution.
5X Digital Zoom is a digitally enhanced zoom. You lose image quality using the digital zoom, where with the optical, you don't lose the quality.
Intelli-Zoom is a technology used that loses very little image quality - so if you use Intelli-Zoom with the 18X optical zoom - you get 36X zoom with very little image quality loss (there still is some - but it is minimized).