Difference between virtual ip and NAT overload

Dear All,

Anybody can explain it  as I am confused a little bit, Explanation should be in sample language with an example.

Since I am new with fortigate NGFW and working with this firewall, I have seen some policy on firewall.

source- all

Destination - virtual ip with mapping such as (external ip - 25.1.1.1 and internal ip 192.168.1.1 )

use dynamic ip pool - NAT with overload .

Regards,

Umesh