Whenever a class itself contains virtual functions or overrides virtual functions from a parent class the compiler builds a vtable for that class. This means that not all classes have a vtable created for them by the compiler. The vtable contains function pointers that point to the virtual functions in that class. There can only be one vtable per class, and all objects of the same class will share the same vtable. 
We have shown that classes without a virtual function indeed contain no virtual pointer and no virtual table is constructed. A virtual table is an array of function pointers although other data types are also possible. The layout is generally compiler-specific (or ABI-specific where multiple C++ compilers share an ABI) and somewhat stable. All the virtual function tables are in the memory associated with your process. In case of GDB all your virtual function tables are stored in read-only memory which protects it from unintentional overwrites. The functions themselves (their assembly instructions) are stored in the .text section of the elf binary.
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