Linear editing is the process of assembling footage in a sequential fashion, from start to finish. This type of editing is generally carried out when working with video tape. Video tape can be cut into sections and spliced together. Instead, the footage is dubbed (recorded) onto a master tape from its various sources and recorded onto the master tape in sequence.
Nonlinear editing is generally carried out on computer software editing facilities. The editor first gathers footage together into the software program then cuts, copies and pastes it together in any order that is desired. When the nonlinear sequence is ready, it is recorded onto the master tape.
Prior to sophisticated computer editing software, film was always edited in a nonlinear style. The film was literally snipped into pieces, and then the pieces were stuck together in any nonlinear order that the editor chose. The process was very slow and greatly limited effects and types of editing transitions, such as dissolves.
Modernized nonlinear editing is digitized, whether the source is video or film. All footage is captured on the computer hard drive before it is transferred shot by shot onto a timeline. Effects and transitions are added if desired, and the editor or director has the choice to change her mind repeatedly about which clip to use, as well as the length of clip. If changes have to be made to linear edited footage, the new clip has to be exactly the same length as the old clip it replaces, and dubbing the new footage in place reduces the image quality.