To understand the significance of the 64-bit version of Office 2013, one must first understand the constraints of its 32-bit predecessor. A 32-bit application is architecturally limited to addressing a maximum of 4GB of virtual address space, which in practice often translates to roughly 2GB of usable RAM for the application itself.
If you already have the 64-bit installer: Office 2013 64 Bits
Office 2013 64-bit can be installed and deployed in a variety of ways, including: To understand the significance of the 64-bit version