What is difference between the index by table and nested tables

Following are the  difference between the index by table and nested tables.

Index by table -

1)Not store in database 

2)Automatically initialize when declared .

3)Index By Table is something like a hash-map or map in another languages where you can index a value with either String or Integer.

Nested table 

1)Store in database 

2)Initialize using contractor .

3)Where are Nested Table is something like Array in another languages but the difference is it is not fixed in size