64 bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs.
I will talk mostly abou the differences between 64 bit OS and 32 bit OS which are mostly used nowadays.
A change from 32bit OS to 64 bit OS was made graduate because the OS must be extensively modified to take advantage of the 64bit architecture in which the binaries can access 264 bytes rather than 232 in the memory. Thus change first has been done with OS that can support a hardware compatibility mode and older software for example a 64 bit OS can have 32bit application through a software emulation.
Example: Microsoft Windows 64 bit also supports software for 32bit and in the task manager there processes are shown with the extension *32
The 64 bit architecture supports large data sets in applications but this compatibility has been debated if the 32bit emulated is slower than a native 32bit OS but so far most of the 32bit application run smoothly on the newest OS.
Another advantage of the 64 bit architecture is that multi-tasking, stress testing and clustering that run better in this type of architecture.There are some misconceptions in that the 64bit architecture is no better than the 32 bit architecture.
For example it is said that the 64 bit architecture is good if you computer has more than 4GB or RAM but as some 32 bit OS shown us that sometimes the 32bit OS can limit you memory to 3GB when the 64bit OS up to 128GB of memory.
Data encryption sometimes is 3 to 5 time faster than the 32 bit architecture and another advantage of the 64 bit architecture is that some tight loops can be performed faster since the processor doesn’t have to get data from cache or other type of memory if the data can fit in the available registers.