Shotgun cloning is the practice of clipping at random a large DNA fragment to reduce it into various smaller pieces that can then be cloned.
The method used to cut the DNA into smaller pieces can be done either through using a restriction enzyme or through more physical methods that have the end goal of smashing the DNA into smaller pieces. The resulting fragments are then gathered and then cloned into a vector. The original DNA can either be a genomic DNA (the process is then called genome shotgun cloning) or a clone like a YAC (yeast artificial chromosomes) that has a large piece of genomic DNA that needs to be split into fragments.
If the DNA is required to be a in a certain cloning vector but the vector is only capable of carrying small amounts of DNA then the shotgun method can be employed. The method is usually used to generate small fragments of DNA for sequencing.
For example, if a geneticist is studying a 50 kb gene it could be difficult to figure out the restriction map. Breaking a DNA sequence into smaller fragments and then mapping these a master restriction map can be deduced.
Web Resources On Shotgun Cloning
Shotgun Cloning & Sequencing
Enotes: Shotgun Cloning
Book Resources On Shotgun Cloning
Condensed Protocols from Molecular Cloning by Joseph Sambrook, David W. Russell
Guide to Molecular Cloning Techniques by John N. Abelson, Melvin I. Simon