You have tagged your question C and C++ at the same time, while the answer is significantly different in these languages.
Firstly, the wording of the title of your question is incorrect (or, more precisely, irrelevant to the question itself). In both of your examples the variable is declared and defined simultaneously, in one line. The difference between your examples is that in the first one the variables are either left uninitialized or initialized with a dummy value and then it is assigned a meaningful value later. In the second example the variables are initialized right away.
Secondly, in C++ language, as @nightcracker noted in his answer these two constructs are semantically different. The first one relies on initialization while the second one - on assignment. In C++ these operations are overloadable and therefore can potentially lead to different results (although one can note that producing non-equivalent overloads of initialization and assignment is not a good idea).
In the original standard C language (C89/90) it is illegal to declare variables in the middle of the block, which is why you might see variables declared uninitialized (or initialized with dummy values) at the beginning of the block and then assigned meaningful values later, when those meaningful values become available.
In C99 language it is OK to declare variables in the middle of the block (just like in C++), which means that the first approach is only needed in some specific situations when the initializer is not known at the point of declaration. (This applies to C++ as well).