If they are connected to your personal or ISP-issued router, then they are considered to be in your network, and might be able to read all of the traffic meant for you.
If they are a neighbor who happens to have the same ISP as you, and use their own personal or ISP-issued router, then they are not in your network, and the only way that they can see any traffic related to your computer is if you sent that information to them.
In most modern setups, your ISP will have a router somewhere in your neighborhood, and will send your traffic directly from their neighborhood router to your own router. If your computer is physically connected to the router, then in most cases, the router will send the traffic directly to your computer. If you have someone malicious who is also physically connected to the router, they can make the switch broadcast your traffic to all ports, rather just the port connected to your computer. If you're both connected through wireless, then you can't assume any privacy, either. However, someone who is connected through wireless is not going to be able to eavesdrop on someone with a physical connection, and vice-versa.
The only way that someone can snoop on your traffic without being connected directly (through a physical wire or through wifi) to your router, is if they have somehow compromised the router in your neighborhood, or have compromised other routers in the ISP's network... but the further away the attack is, the less of their traffic they'll be able to intercept.