Dideoxy sequencing is a lab technique also known as Sanger sequencing (it was invented by a scientist named Fred Sanger). It is used to find out the sequence of nucleotide bases in a DNA molecule.
The technique involves putting the DNA molecule and the components needed to replicate it into each of four test tubes, along with one of the four types of dideoxynucleotides (ddA, ddT, ddC, ddG). Dideoxynucleotides are molecules that cause the premature termination of DNA replication at the base that they substitute for. The result is that each of the test tubes contains fragments of the DNA molecule that all end at the same base, but at different points on the molecule where the base occurs.
The contents of the test tubes are then separated by size with gel electrophoresis (one gel well per test tube; four total wells); the smallest fragments will travel the farthest and the largest will travel the shortest distance from the well. The sequence can then be determined from the picture of the finished gel by noting the sequence of the marks on the gel and from which well they came from.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.