one transaction or multiple inserts?

When inserting a lot of data into a DB which would be faster, doing all
the inserts in one transaction or doing multiple inserts?

And more importantly, why? :)

I'M assuming that each insert is in fact a transaction and doing, for
example, 1000 inserts is the same as doing 1000 transaction. But if I
put them all in one transaction, it would be equivalent to doing (duh) 1
transaction.

Thanks,

Jc