Polymerase enzymes are responsible for replication and formation of new nucleic acid strands. Polymerase enzymes exist in many forms, and fulfill different roles in molecular biology based on template and end product:
DNA-dependent DNA polymerases (often referred to as DNA replication enzymes or simply DNA polymerases) recognize a template of single stranded DNA, and anneal one deoxynucleotide at a time to form a complementary strand of DNA. DNA-dependent DNA polymerases are most frequently used for DNA replication in experiments like polymerase chain reaction (PCR).
RNA-dependent DNA polymerases (often referred to as reverse transcriptases) recognizes a template of single stranded RNA, and anneal a single deoxynucleotide at a time to form a complete complementary strand of DNA. RNA-dependent DNA-polymerases are frequently used to generate cDNA copies of mRNA transcripts (e.g. when generating a cDNA library via RT-PCR).