SeriesData[x, $x_0$, {$a_0$, $a_1$, ...}, $n_{min}$, $n_{max}$, den]SeriesData is the Head of expressions generated by Series:
series = Series[Cosh[x],{x,0,2}]
Head[series]
series // FullForm
You can apply certain mathematical operations to SeriesData objects to get new SeriesData objects truncated to the appropriate order.
series + Series[Sinh[x],{x, 0, 3}]
Series[f[x],{x,0,2}] * g[w]
The product of two series on the same neighborhood of the same variable are multiplied:
Series[Exp[-a x],{x,0,2}] * Series[Exp[-b x],{x,0,2}]
D[Series[Exp[-a x],{x,0,2}],a]
See also Series and SeriesCoefficient.