A General Overview of Eavesdropping

Communications traveling over the internet are routed through many intermediate computers, called routers, which direct messages as they are sent by the programs that send them. If the programs that send them send all or part of those messages in an unencrypted format or in an encryption format known to be breakable those routers can intercept and copy those messages as they are routed, if necessary decrypting them at their leisure.

But NOTHING is encrypted unless the program tells it to be

Also, the NSA isn't compromising home computers directly, it is monitoring them by compromising routers and stealing the messages they communicate.