DNA replication is the process by which new copies of DNA are produced, and it normally occurs prior to cell division.
DNA is made of two complementary chains or strands, and during replication, each of these strands is used as a template for the synthesis of a new strand. This means that each molecule of DNA produced in replication has one "old" strand and one "new" strand, and is because of this that replication is considered a semi-conservative process.
Given that DNA has a double helix structure, this helix must be unwound to allow replication to occur. This unwinding is performed by an enzyme called helicase, and after that, the DNA polymerase proceeds to add the complementary nucleotides to each DNA strand.
