During elongation, RNA polymerase slides down the double stranded DNA, unwinding it and transcribing (copying) its nucleotide sequence into newly synthesized RNA. The movement of the RNA-DNA complex is essential for the catalytic mechanism of RNA polymerase. Additionally, RNA polymerase increases the overall stability of this process by acting as a link between the RNA and DNA strands.  New nucleotides that are complementary to the DNA template strand are added to the 3 end of the RNA strand. The newly formed RNA strand is practically identical to the DNA coding strand (sense strand or non-template strand), except it has uracil substituting thymine, and a ribose sugar backbone instead of a deoxyribose sugar backbone. Because nucleoside triphosphates (NTPs) need to attach to the OH- molecule on the 3 end of the RNA, transcription always occurs in the 5 to 3 direction. The four NTPs are adenosine-5-triphosphate (ATP), guanoside-5-triphosphate (GTP), uridine-5-triphosphate (UTP), and cytidine-5-triphosphate (CTP). The attachment of NTPs onto the 3' end of the RNA transcript provides the energy required for this synthesis. NTPs are also energy producing molecules that provide the fuel that drives chemical reactions in the cell.
Multiple RNA polymerases can be active at once, meaning many strands of mRNA can be produced very quickly.  RNA polymerase moves down the DNA rapidly at approximately 40 bases per second. Due to the quick nature of this process, DNA is continually unwound ahead of RNA polymerase and then rewound once RNA polymerase moves along further.  The polymerase has a proofreading mechanism that limits mistakes to about 1 in 10,000 nucleotides transcribed. RNA polymerase has lower fidelity (accuracy) and speed than DNA polymerase. DNA polymerase has a very different proofreading mechanism that includes exonuclease activity, which contributes to the higher fidelity. The consequence of an error during RNA synthesis is usually harmless, where as an error in DNA synthesis could be detrimental.
The promoter sequence determines the frequency of transcription of its corresponding gene.