What is a 64 bit computer ?

What is a “64-bit” computer? 

            

      
The word size of a computer generally indicates the largest  integer it can process in a single instruction, and the size   of a memory address, which is usually, but not necessarily  the same as the integer size.       
                  

The main indication of the word size is how much memory the processor can address.  A 32-bit processor is limited to        232 memory addresses, each of which   usually holds one byte.  Hence, 32-bit PCs and Macs are limited to a maximum of 4 gigabytes of electronic memory (RAM and ROM).       
                        

Note that 32-bit processors such as the 486, Pentium, and PowerPC G4 have supported 64-bit floating point numbers for a long time, but were still regarded as 32-bit processors.