1 Answer

Thermocycler - This is where the reaction is going to take place
Polymerase - Usually Taq polymerase is used, but other ones (such as Pfu) can be used as well
dNTP (deoxynucleotide triphosphate) - dATP, dTTP, dGTP, dCTP (These are the 4 nucleic acids in DNA attached to a ribose sugar with Triphosphate on the 5' end)
Primers - Forward and reverse primers are needed to ensure replication of both strands
MgCl2 - This acts as a cofactor for your polymerase, stabilizing it
Target DNA - This is what you want to have amplified, you generally want a few microliters of this.

That's pretty much all you need to perform basic PCR. If you're doing something like qPCR, or real time PCR, you need more things (some sort of probe) or if you're doing RT-PCR you need reverse transcriptase as well. Otherwise, traditional PCR just needs the above things. Usually you can buy a buffer solution that contains all your nucleotides and salts already.
Source:
Molecular Biology Major http://answers.yahoo.com/question/index?qid=20100225122959AAQ6htg