A Network has a different set of nodes, validators, systems, etc, and usually tokens that are used for pay the gas fees for it.
Some projects decide to go for one network or another based on the gas fees or features.
There are several known networks like:
| network |
|---|
| Ethereum |
| Cosmos |
| Aleph |
| Gnosis/xDai |
| Binance Smart Chain |
| Polygon (Matic) |
And many projects based on that ecosystem