What is Generator?

When should one use it? I did go through the documentations but as my
english is weak I am not able to understand everything about it.

What is the best way of creating an AutoIncrement numeric field in FB?

Will Generator always generate a unique value which will not get
repeated or can the value generated by a Generator get duplicated?

Please advise as I want to implement a few tables each having one Auto
Increment field. The value in each Auto Increment field the value should
be unique across database not only withing a table.

TIA

Yogi Yang