To clarify:
For instance, the difference of building an in-game team or a NB team.
An in-game team takes your time and devotion to build. Like being an actual trainer, you have to breed, calculate IVs and EVs etc etc
A Netbattle team is put together in mere minutes. You can choose whichever pokémon you like and have it use any attack that it can learn without you being forced to go through hours of breeding and/or training to obtain them.