Tooltips
This is where you can show help to the user. Naturally you don't want this help to be permanently visible on the screen so you can hide it and bring it up when needed.
Move over question mark for help ?
Move over this text to see a tooltip with a delay added.
Oh yes. Move over this for a delay and a fade.
For tabulated data (especially when you have lots of fields and need space)
| Animal | Type | Where to find |
| Cat | Mammal | Sleeping on the TV |
| Tuna | Chicken ;) | In the cupboard |
| Spider | Insect | Behind you |
| Chicken | Poultry, haha | Crossing the road |
| Lama | Mammal | Really slow fading tooltip |
For popups appearing after user clicks a link
The above uses the "requireclick" property of the tooltip.
The idea was inspired by a post in the forum.
On Images
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Plain description 
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Image maps
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When space is tight (images in BoxOver)
I know this is a little bit of a lame example but I needed a fourth to fill up this table ;)
Move over thisYeah...about that tight space.