What is a SPATIAL INDEX and when should I use it?

Like most of the average PHP web developers I use MySql as a RDBMS. MySql (as other RDBMS also) offers SPATIAL INDEX features, but I'm don't get it very well. I have googled for it but didn't find clear real world examples to clarify my bad knowledge about it.

Could someone explain me a little bit what is a SPATIAL INDEX and when should I use it?

You can use a spatial index for indexing geo-objects - shapes. The spatial index makes it possible to efficiently search for objects that overlap in space

Spatial Index is like an ordinary index with this difference that Spatial objects are not 1D data points rather are in higher dimension space (e.g. 2D) and thus Ordinary indexes such as BTree are not appropriate for indexing such data. The well-known spatial Index technique is R-tree ( Google it on wikipedia )