series.str.cat is the most flexible way to approach this problem:
For df = pd.DataFrame({'foo':['a','b','c'], 'bar':[1, 2, 3]})
df.foo.str.cat(df.bar.astype(str), sep=' is ')
>>> 0 a is 1
1 b is 2
2 c is 3
Name: foo, dtype: object