In [17]: from pandas import Series,DataFrame
In [18]: series1 = Series(range(2),index = ['a','b'])
In [19]: series2 = Series(range(3),index = ['c','d','e'])
In [20]: series3 = Series(range(2),index = ['f','g'])
In [21]: import pandas as pd

In [22]: pd.concat([series1,series2,series3])
Out[22]: 
a 0
b 1
c 0
d 1
e 2
f 0
g 1
dtype: int64

In [24]: S1=pd.concat([series1,series2,series3],axis=1)
In [25]: S1
Out[25]: 
 0 1 2
a 0.0 NaN NaN
b 1.0 NaN NaN
c NaN 0.0 NaN
d NaN 1.0 NaN
e NaN 2.0 NaN
f NaN NaN 0.0
g NaN NaN 1.0
In [26]: type(S1)
Out[26]: pandas.core.frame.DataFrame

In [27]: type(pd.concat([series1,series2,series3]))
Out[27]: pandas.core.series.Series

In [29]: series3 = Series(range(2),index = ['f','e'])
In [30]: pd.concat([series1,series2,series3])
Out[30]: 
a 0
b 1
c 0
d 1
e 2
f 0
e 1
dtype: int64