Suet is raw hard beef fat, especially the hard fat found around the loins and kidneys. It is a saturated fat and usually used to render into tallow. Tallow is generally used for candles (in the old days) or frying. Personally, I prefer to use it over pork fat (lard) in terms of flavor.
If you want to compare the tastes, compare beef and pork bacon. Both are great but the flavor is very different.
Suet, until it is rendered, must be kept refrigerated. Its primary purpose in the US is to be rolled in bird seed, placed in a mesh bag , and placed out in the winter months for birds that remain in the area.
As you can tell, we were very familiar with suet as my neighbor was a grocery manager.
It is very hard to find these days as most of the beef sent to grocery stores is box beef that would exclude items such as suet. I have seen it on occasion at the Woodman's store on Randall Rd. in Carpenterville. I would STRONGLY suggest calling the butcher out there before making the drive.