nullptr is always a pointer type. 0 (aka. C's NULL bridged over into C++) could cause ambiguity in overloaded function resolution, among other things:
f(int);
f(foo *);
Here is Bjarne Stroustrup's wordings,
In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days.
If you have to name the null pointer, call it nullptr; that's what it's called in C++11. Then, "nullptr" will be a keyword.
You can find a good explanation of why it was replaced by reading A name for the null pointer: nullptr, to quote the paper:
This problem falls into the following categories:
Improve support for library building, by providing a way for users to write less ambiguous code, so that over time library writers will not need to worry about overloading on integral and pointer types.
Improve support for generic programming, by making it easier to express both integer 0 and nullptr unambiguously.
Make C++ easier to teach and learn.
One reason: the literal 0 has a bad tendency to acquire the type int, e.g. in perfect argument forwarding or more in general as argument with templated type.
Another reason: readability and clarity of code.
Source: Stackoverflow.com