Where physical won’t work
Virtual caches should only be placed in areas where physical caches are not allowed or otherwise unfeasible. If a physical cache is possible at the location, then hide a physical geocache there, not a virtual.
Just getting there isn’t enough
Geocaching should get you off your couch and out of your car. Virtual caches must include a search, seek, or do activity at the coordinates, ideally with an educational, outdoor, or human interest component. This activity can involve finding the code phrase as discussed below.
You still have to follow the rules
Virtual caches must still respect private property, local regulations so good judgment. Just because you aren’t hiding a physical box doesn't make it ok to list a virtual cache on private property without the owner's permission or encourage behavior that is not beneficial to the geocaching community.
Keep it close to home
Like physical caches, you should add virtual caches in places your frequently visit. Put a virtual cache somewhere where you can offer the finder a unique insight into the area.
The code phrase
Since virtual caches don't have a physical logbook, so some cache owner will ask that you provide a code phrase to prove you found the virtual cache. The cache description will have instructions on how to determine the code phrase when you get to ground zero.
The phrase could be anything. Maybe the last two words inscribed on a monument, or the number of pillars that support an old building. It’s up to the geocache hider to be creative.