DNA polymerase I is an enzyme that aids in DNA replication. It was discovered by Arthur Kornberg, in the mid 1950's, and was the first such enzyme discovered (hence the name). It is often referred to as Pol I, for short.
DNA polymerase I removes the RNA primer from the lagging strand and fills in the necessary nucleotides (see DNA replication) in 5' -> 3' direction. Ligase then joins the various fragments together into a continuous strand of DNA.
DNA polymerase I obtained from E. coli is used extensively in molecular biology. The holoenzyme has two exonuclease activity domains: one 5' -> 3' and one 3' -> 5', this last one being known as the large fragment or Klenow fragment when the holoenzyme is cleaved by proteolysis.