Simulation (logic simulation)
Beginner
Running the chip design as a program on an ordinary computer, to see what the chip would do.
Novice
Running a software model of the design on an ordinary computer. The simulator works out the value of every wire, clock cycle by clock cycle, while a test feeds in inputs. It is far slower than a real chip, but any signal can be inspected at any moment.
Expert
The simulator interprets or compiles the RTL into a program that computes every signal each cycle. It can run any testbench code and show any signal, but it runs far slower than silicon, and slower still as the design grows, which is why very long software runs move to emulation.