Fourier[list]
InverseFourier[list]

N1=256
Pure sin wave (note positive and negative frequency):
data = Table[N[Sin[20 2 Pi k/N1]], {k, N1}];
ListLinePlot[data]
Data=Fourier[data];
ListLinePlot[Abs[Data],PlotRange->All]

sin wave + noise: see small amplitudes (Data) at all frequencies
data = Table[N[Sin[20 2 Pi k/N1] + (RandomReal[] - 1/2)], {k, N1}];
ListLinePlot[data]
Data=Fourier[data];
ListLinePlot[Abs[Data],PlotRange->All]

sawtooth (has discontinuity -> lots of strong harmonics)
data = Table[N[Mod[k,20]], {k, N1}];
ListLinePlot[data]
Data=Fourier[data];
ListLinePlot[Abs[Data],PlotRange->All]

Data[[1]]
this is the zero^th harmonic, i.e, the DC signal
to remove DC subtract average:

md=Mean[data]
data = Table[N[Mod[k,20]-md], {k, N1}];
ListLinePlot[data]
Data=Fourier[data];
ListLinePlot[Abs[Data],PlotRange->All];
Data[[1]]

triangular wave...no discontinuity so weaker harmonics
data = Table[Abs[N[Mod[k,40]-20]]-10, {k, N1}];
ListLinePlot[data]
Data=Fourier[data];
ListLinePlot[Abs[Data],PlotRange->All];

smooth the sawtooth with running average:
data = Table[N[Mod[k,20]-md], {k, N1}];
data2=Table[Sum[data[[k1]],{k1,Max[1,k-5],k}]/5,{k,N1}];
ListLinePlot[data2];
Data2=Fourier[data2];
ListLinePlot[Abs[Data2],PlotRange->All];

smooth the sine wave+noise with running average
data = Table[N[Sin[20 2 Pi k/N1] + (RandomReal[] - 1/2)], {k, N1}];
data2=Table[Sum[data[[k1]],{k1,Max[1,k-5],k}]/5,{k,N1}];
ListLinePlot[data2];
ListLinePlot[data];
Data2=Fourier[data2];
ListLinePlot[Abs[Data2],PlotRange->All];
Data=Fourier[data];
ListLinePlot[Abs[Data],PlotRange->All]