"cDNA or complementary DNA" has nucleotide sequence complementary to the processed "mRNA from which it has been synthesized in a process called reverse transcription that is catalyzed by enzyme i.e reverse transcriptase.
cDNA being synthesized from single stranded "mRNA" is firstly synthesized as a single stranded molecule. And it remains single stranded until "PCR(polymerase chain reaction)" is carried out. In PCR that is aided by "DNA polymerase" (e.g Taq DNA polymerase) , it gets it's double stranded form.
http://evrogen.com/technologies/Mint-cDNA-synthesis.shtml
Hope it helps...