What exactly does it mean when a carrier uses another telco's network?

http://ift.tt/2DpbCVj <-- Example http://ift.tt/2ChokGx <-- Virtual networks?

If X company uses Y companies network, does it mean that X company will get the full coverage and speed of the Y network?

The same coverage and speed that Y customers would get?

Or would X company be handicap in some way?