Python supports arbitrary precision integers, meaning you’re able to represent larger numbers than a normal 32 or 64 bit integer type. The L tells you when a literal is of this type and not a regular integer.
Note, that L only shows up in the interpreter output, it’s just signifying the type. If you print that result instead:
>>> print(25 ** 25)
88817841970012523233890533447265625
The L doesn’t get printed.
In Python 3, these types have been merged, so Python 3 outputs:
Python 3.4.0 (default, Apr 11 2014, 13:05:11)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 24 ** 24
1333735776850284124449081472843776