What are the differences between a declaration and a definition?

There are two differences between a declaration and a definition:

1. In the definition of a variable space is reserved for the variable and some initial value is given to it, whereas a declaration identifies the type of the variable.
2. Redefinition is an error, whereas, redeclaration is not an error.

Example of Definition:

int a=10;

char  ch='A';

Example of Declaration:

extern int a;

extern char ch;