Just as there are different types of maps of the earth, geneticists use different maps to express different genetic information.
A physical map of the genome is like a road map. It shows the distance of a specific base pair from a reference point known as an STS, or Sequence Tagged Site. This is accomplished by cutting up the genome and then figuring out how the fragments fit into existing STS maps. It is a high-tech approach in which the sequences of bases in the DNA must be determined.
A genetic-linkage map shows the order of genes on a chromosome, and is a way to visualize how and where crossing over has occurred in meiosis. This can be done through classical genetics studies, by looking at how traits that come from adjacent genes assort from one generation to the next; it does not require knowing the nucleotide base sequences that make up the gene.