Introduced in about 1999, they come in 3 flavours: weblocs, maillocs and textClippings. How they have changed, how you can still use them, and how to fix them.
webloc
Safari 12.1.1 will write a folder of weblocs from its Bookmarks page, but forgets to close and release those files. Look at the problems which result.
The Eclectic Light Company 