Edit
A comical depiction of crying or tears. Very similar to sweatdrop. This tag is not meant to specify when there's one tear versus when there's more than one. Usually there is never more than one on a character's face.
Basically, teardrop is used when the tear is drawn on for comedic purposes, rather than to actually represent happiness/sadness/pain/pleasure/etc. They usually have a specific color, rather than being transparent like real tears, and are usually connected directly to the eyes. A teardrop may also accompany yawning.
Rule of thumb: if the character appears to be actually crying, or the teardrop is transparent, it is usually not a teardrop. For all other instances of shedding tears that don't properly meet the requirements for teardrop, use tears instead.
For non-comedic pictures with a single teardrop, use single tear.