Please read the following the best explanation from one of the best engineers on Cisco TAC of Belgium Back Bone Marcin Latosiewicz:
enjoy:
Understanding ASA IPSec and IKE debugs - IKEv1 Main Mode.
https://supportforums.cisco.com/docs/DOC-14044
Understanding ASA IPSec and IKE debugs - IKEv1 Aggressive Mode.
https://supportforums.cisco.com/docs/DOC-13715
