By default the Opp Lead Source is as Hanne describes: the lead source of the lead/contact from whom the opp was created.
Some companies go a step further and create something called an "Opportunity Source" field that's meant to give credit to "who qualified the lead". That way on each opp, you'll have the "Lead Source", say a trade show, and an Opp Source, which could be "Partner, Inside Sales Team, Inbound Call, etc, etc, etc)".
Just food for thought.