A voice-over is a narration done by usually reading from a script prepared beforehand. The narrator's voice is recorded over a sequence of video clips that tell a story.
Generally you will write the script after shooting the video, or still photographs for a digital story, and then assemble the clips to fit the script. Most of the time, your voice over is recorded prior to assembling and editing the photos and video.
Reading from a script is an art that requires both smooth delivery and emphasizing the key points in the story. Here are some tips for doing these voice-overs.