When I try to explain multi-tenancy to my customers, I always compare it to an apartment block.
For example, consider a scenario, where you – as a company or a customer – rent an apartment in a block that is owned by Salesforce, who is your landlord:
Here, your apartment has specific layouts and resources – that is, it has a number of rooms divided by walls. In addition to this, it has central heating, electricity, water, and more. To access and use this apartment, you pay a monthly rent, and everything else is taken care of for you and the other occupants in the building by your landlord.
Apart from your apartment (which is your private space), all the other resources are shared by the occupants of the building. This means that if Salesforce decides to upgrade the central heating to underfloor heating, then you will automatically benefit...