|Image from here.|
Once upon a time, computer monitors were big. You may remember... They looked like the ones we have now, with a gigantic box on the back. ;) Anyway, you may have experienced the screen flickering when your speakers were too close to it, or a power strip or something like that. This is EMI. The magnetic fields generated by the power strip, or the actual magnets in the speakers could cause distortion in the picture tube.
Likewise, I had a set of speakers that used to produce a sort of flickering sound when someone with a Nextel phone was about to get a call. Not sure if this was technically electro-magnetic interference, but same idea, I think...
I've seen modems and routers stop working because they were too close to the power adapter... Like, the one that CAME WITH them. If the power strip was too close, the user would be kicked off the internet.
EMI is why we can't use our electronics on air planes. EMI is why cell phones are not allowed in hospitals. Not that they DO or WILL cause problems, but that they MIGHT... and it's not a risk worth taking.
These days most things are "shielded" and aren't as impacted by the EMI fields.
There's so much flying around in the air these days, WIFI signals, radio signals, cellular signals, etc... This is why I NEVER recommend tape backups, and I am so excited that they finally seem to be on their way out. I hate that they've been the industry standard for so long.
What does a magnetic field do to a tape? It degrades the information on it, and sometimes completely erases it. Well, my friends, the Earth is a GREAT BIG MAGNET. This is why tapes have a shelf life.
One last thought: We really have no full idea of the effects of these EMI fields on our persons. One wonders...