A DNA polymerase is an enzyme which makes DNA molecules from its nucleotide building blocks. DNA polymerases are essential for DNA replication. They usually work in pairs as they copy one double-stranded DNA molecule into two double-stranded DNAs. In DNA replication DNA polymerase "reads" a piece of DNA that's already there and uses it to make a new piece that is exactly the same as the old piece.
DNA polymerases also play key roles in other processes within cells, including DNA repair, genetic recombination, reverse transcription, and antibody production. DNA polymerases are widely used in molecular biology laboratories, notably for the polymerase chain reaction (PCR), DNA sequencing, and molecular cloning.