How rapidly a particular DNA polymerase enzyme synthesizes a complementary strand.
A measure of how strongly the polymerase binds to template DNA. Processivity doesn't affect extension rate. However, it has a strong influence on the absolute length of complementary DNA strand that can be made.
Proofreading is when the multi-functional enzyme adds a deoxynucleotide and then goes back and rechecks the template to complement interaction to make sure the correct nucleotide was added.
Proofreading polymerases are usually multimeric and thus more expense to make.
Like all DNA polymerases, proofreading polymerases add an adenine deoxynucleotide to the 3' end of the complementary strand. Proofreading ensures that this nucleotide is removed again leaving blunt-ended DNA.