Both 网上 and 在线 mean "online". What is the difference between 网上 and 在线?
Specifically, although “网上” and “在线” can be both translated as "oline", “网上” actually means "on the internet", “在线” means "online".
Here are some examples
It means, I found this article online.
我的 QQ 是在线的。
This means, the status of my QQ is online.
Although they were both translated to "online", I bet you can see the difference there with actual examples.