An omniscient narrator can see inside all characters and tries to relate all of the information in a way so all sides can be seen. The narrator and see everything and also knows everything.
An unreliable narrator does not have the credibility to be trusted when telling the story. This could be due to age, either young or old, mental capacity or personal motivation. The unreliable narrator is best used in irony, as the narrator seemingly is telling the truth (as far as the narrator sees it) but through actions, the narrator sets him/herself up in a situation that shows the irony of what the narrator sees as true.