It is hard to entirely explain via this forum. Start by reviewing the Cisco Optical Networking web pages:
<http://www.cisco.com/en/US/products/hw/optical/index.html>
The ONS series is designed to aggregate various traffic types (video, voice, data, SAN) for transport across an optical network. Its main customers are service providers and large enterprise companies.
For service providers, the equipment is typically installed in the central offices. The ONS devices are interconnected via fiber and aggregate traffic from the various equipment used in these facilities including routers, carrier Ethernet switches, SONET/SDH multiplexers, voice switches, M1/3 multiplexers, channel banks, cross-connects, etc. to be combined onto a common optical infrastructure. The ONS 15454 MSTP is a DWDM platform, allowing more bandwidth to be carried over a fiber; in the form of wavelengths (channels) of light.
For large, multi-location enterprise customers, they typically install the ONS devices in their data centers to interconnect their offices so as to transport their video, voice, data and SAN traffic. They may interconnect these offices over their own dedicated fiber or hand-off to a carrier/service provider.
Here is one example for how the platform was used by Cisco IT in San Jose:
<http://www.cisco.com/web/about/ciscoitatwork/network_systems/optical_migration_for_tdm_local_access.html>
You might review some of the case studies to see how other customers are using the platforms.
Good luck!