Oracle Database Discussions

Can somebody tell me what is the unit of 'cost' which we see in the explain plan of a query. I know there can be tree types of costs. (1) CPU usage, (2) Memory usage and (3) Disk I/O.

Now when we see the explain plan of a query, it shows the cost of each step in the plan, my question is "in what terms is this cost specified?" is it cpu cost? or memory cost? or disk I/O cost?.

I have also noticed that sometimes a stem with higher cost is completed earlier than a step with lower cost. what does this mean?

Thanks.