Network Sharing - Cell Site Sharing, MOCN, MVNO
Network Sharing is a method of sharing some portions of network architecture among multiple parties. Of course, the major motivation of network sharing is to save the cost or save the critical locations. There are many types (levels) of network sharing. Depending on which portions of the network are shared, they use different terms.
Cell Site Sharing : I don't think I need to explain about this. As it stands for, Cell Site Sharing is just to share a location (real estate).
MOCN : MOCN stands for Multiple Operator Core Network. Basic idea of this is to share Access Network part (NodeB, eNodeB) between two or more network operators. Sharing NodeB/eNB usually implies sharing Cell Site as well. In MOCN, the sharing parties tend to be similar scales (like each of the parties are independent Network Operators). Usually each of these operators has some existing area where they have their own NB/eNBs but try to share these resources in those area where the service area of the Network operators are overlapping.
In this type of sharing, they use USIM with multiple PLMNs and MIB (in case of 3G) is configuring the multiple PLMN (each PLMN belong to each sharing Operators).
MVNO : MVNO stands for Mobile Virtual Network Operator. Strictly speaking this may not be regarded is a 'Sharing'. In this mode, a service provider does not have their NB/eNB at all. They are just renting NB/eNB from a Network Operator. Usually small service provider (like prepaid phone service or providing a special service only). Recently (as of Sep 2015) I am seeing multiple MVNOs providing IoT/M2M related service only. Usually these operators issues their own USIM.
The typical implementation of MOCN can be illustrated as below. As you see here, core network from two different network operator shares same eNB/BTS. The eNB/BTS is transmitting multiple PLMN in SIB 1(in case of LTE or NR) and MIB (in case of WCDMA). UE is connecting to different corenetwork depending on which PLMN it camped to.