Slew bearings, Slewing bearings, and cross-roller bearings are all the same thing. The unique advantage is that they can take forces in every direction. Some bearings you can pop out the center if you push wrong. Some you can't use like a car wheel. Some you can't put a pencil in the hole and bend. Slew bearings can take a (relatively) large amount of all these forces, and do it in a really low profile (thin design).
I wanted a slew bearing for my robot. One slew bearing can replace about 6 parts in my current design and takes up a lot less room. Online slew bearings cost $120 before $60 shipping. No thank you! I got 100pc 1/8" ball bearings from Aliexpress for $10. This is a job for my 3D printer.
PS: There are four steps to this instructable but only 3 are displayed on the main page. Step 4 has the STL files and more.
Step 1: Design
I searched for "slew bearing cross section" and found http://www.sbibearing.com/userfiles/zp01.JPG
I modified the design to suit my tastes. The inside is two identical pieces. In the picture the inside and outside sit flush. In the photos I didn't plan so well and I had to chase bearings all over the place during assembly.
Based on the design I knew I would need 70 balls. In the end I used 68 to leave a bit more room for movement.
Dear Solidworks: please come out with a $500 edition for hobbyists. Sincerely, everybody.