Difference between db->insert and db->insert_string

[eluser]rahendz[/eluser]
i'm just curious, first sorry for my bad english

1. is there any difference between insert function with insert_string function?

2. could i retrieve insert_id when running insert_string like insert function?

3. could i do something like this

Code:

return $this->db->insert($table,$data)->insert_id();

thanks before.