Here's what shuts down
National Parks
Monuments
Smithsonian Museums
Processing of applications for passports/visas
New enrollments in experimental treatment at the NIH (National Institute of Health)
Maintenance of US government websites
Mortgage approvals are delayed
Approval of drilling applications at the Bureau of Land Management
Small Business loans
Start of the Alaska crab season
(It relies on harvest levels apportioned by NOAA-the National Oceanic and Atmospheric Administration)
Employees are placed on unpaid furlough. When they return they are not guaranteed retroactive pay but always have gotten it in the past....eventually.