For example, I always referred to the cards that come inside the jewel case of a cassette as an "insert" or "liner notes." Both wouldn't be necessarily correct or incorrect, but there are more accurate ways to refer to them. Sure, it is an insert in that it sits inside the case. Sure, they can be liner notes, but that's not always the case. Sometimes they're blank or they contain advertising for the recording company. "Liner notes" is almost too specific. 

One of the more accurate ways to refer to them is alphabetically. They are:

J Cards - this is the most standard of insert card types. It is so named because you can turn it and look at it from the end and it forms the letter J.

The Traveling Wilburys - Vol. 1

U Cards - this is a variation of the J card where the end that is usually stubbier is the same length as the other end and forms the letter U.

The Black Crowes - Shake Your Money Maker

O Cards - this is a full-wrap card that doubles as the container for the tape since most O cards do not have a jewel case. It forms the letter O (or, arguably, D) and they were used primarily for cassette singles (“cassingles”).

INXS - “New Sensation” single

Kinda cool, eh? So now you know. And knowing, as G.I. Joe taught me in the 1980s, is half the battle.