A herm is a plant that grows (a lot of) both male and female flowers - an actual hermie is pretty easy to spot
Nanners are usually solitary male flowers that will occasionally (and randomly) show up on a female plant, usually real late in the game- in other words, she's tired of waiting, so she decides to do it herself...
When they open, they look like a tiny bunch of bananas- so "nanners"
They can also just show up randomly on a younger plant-these are the dangerous ones because they're hard to spot, and can pollinate the whole plant (or plants) if you don't catch them before they open.
In my experience, this doesn't happen very often though.
Nanners are usually solitary male flowers that will occasionally (and randomly) show up on a female plant, usually real late in the game- in other words, she's tired of waiting, so she decides to do it herself...
When they open, they look like a tiny bunch of bananas- so "nanners"
They can also just show up randomly on a younger plant-these are the dangerous ones because they're hard to spot, and can pollinate the whole plant (or plants) if you don't catch them before they open.
In my experience, this doesn't happen very often though.