DNA helicases are molecular 'motor' enzymes that use the energy of NTP hydrolysis to separate transiently energetically stable duplex DNA into single strands. They are therefore essential in nearly all DNA metabolic transactions. They act as essential molecular tools for the cellular machinery. Since the discovery of the first DNA helicase in Escherichia coli in 1976, several have been isolated from both prokaryotic and eukaryotic systems. DNA helicases generally bind to ssDNA or ssDNA/dsDNA junctions and translocate mainly unidirectionally along the bound strand and disrupt the hydrogen bonds between the duplexes. Most helicases contain conserved motifs which act as an engine to drive DNA unwinding. Crystal structures have revealed an underlying common structural fold for their function. These structures suggest the role of the helicase motifs in catalytic function and offer clues as to how these proteins can translocate and unwind DNA. The genes containing helicase motifs may have evolved from a common ancestor. In this review we cover the conserved motifs, structural information, mechanism of DNA unwinding and translocation, and functional aspects of DNA helicases.