What is the function of NFFT in MATLAB code of the frequency plot?

Hey

I have made a Frequency plot, according to the instructions on the site "MATLAB: fft". >> L=length(channel1'); >> NFFT=2^nextpow2(L); >> Y=fft(channel1,NFFT)/L; >> f=fs/2*linspace(0,1,NFFT/2+1); >> plot(f,2*abs(Y(1:NFFT/2+1)))

But there are a couple of things I don't entirely understand. What is function of the NFFT? And what exactly is the measure of the Y(f)? Sometimes the Y(f) is described as the Magnitude, other times as the dB/Hz or watt/Hz? What exactly is the difference?