Update: df.get_value is deprecated as of version 0.21.0. Using df.at or df.iat is the recommended method going forward.
df.at can only access a single value at a time.
df.loc can select multiple rows and/or columns.
Note that there is also df.get_value, which may be even quicker at accessing single values:
In [25]: %timeit df.loc[('a', 'A'), ('c', 'C')]
10000 loops, best of 3: 187 µs per loop
In [26]: %timeit df.at[('a', 'A'), ('c', 'C')]
100000 loops, best of 3: 8.33 µs per loop
In [35]: %timeit df.get_value(('a', 'A'), ('c', 'C'))
100000 loops, best of 3: 3.62 µs per loop
Under the hood, df.at[...] calls df.get_value, but it also does some type checking on the keys.