North Dakota State University defines a DNA probe as a small piece of nucleic acid (typically single-stranded DNA) that is used to detect complementary stretches of DNA. DNA probes used in hybridization experiments are labeled so that they can be visualized.
As Wikipedia explains, DNA probes are typically labeled with either radioactive or fluorescent bases. It is more common to use fluorescent DNA probes because a variety of different fluorescent wavelengths can be chosen, which allows scientists to hybridize more than one probe to a given sample at a time.
According to "Human Molecular Genetics," DNA probes can be used for a variety of different molecular biology techniques. When used with a Southern or northern blot, DNA probes can be used to detect size fragments of a gene or other segment of DNA in question.
DNA probes can also be used for fluorescence in situ hybridization techniques, in which chromosomes are denatured on microscope slides. This method allows for the detection of larger chromosomal aberrations, including broken or fused chromosomes. Chromosome painting techniques utilize in situ hybridization to paint metaphase chromosomes different colors, which makes chromosome mapping easier. This technique can also be used to detect chromosome sequences that have been conserved between different species.