A detail involves working in a different office or job for a specified amount of time. When working on a detail, you are still assigned to your regular duty station, but for a period of time, you work for a different supervisor. During your detail, you will be assigned jobs to do based on the position you are filling.
Examples of details:
Your permanent job with the Service may be as an executive assistant in the Regional Office. You and your supervisor have determined that it would be a good idea for you to "experience" what it's like in the field. You know of an administrative technician at a Refuge who wants to come to the RO for a few weeks to see what it's like to work there. You and the admin tech may decide to switch jobs for 2 weeks, so you agree to go there and he agrees to come to the RO. During this time you continue to do "real" work, it's just at a different location and for different people.
You are an ES biologist and want to learn about how the budget process works in the Service. You talk to the Budget Chief in Washington and you agree to go work there for 2 months. You go to Washington and work for them during that time.
There are lots of different things you can do for a detail. Be creative, find a place where they need someone with your skills, and see if it can work out. It can be a great learning experience - especially to help you find out if a particular job would be "for you."