Terminology

Subunit: A smaller group formed with a subset of the main group's members that often use the initials of the names of the subunit's members to form the group name.
Misamo: Twice's only current subunit, and the name was derived from the initials of the three Japanese members, Mina, Sana, and Momo.
Comeback: This is just the word used to describe upcoming new music from a kpop artist. Example: "I'm excited for Twice's next comeback! I hope it includes more solo songs!"
Season's Greetings: These are special packages that generally include a calendar, and sometimes a journal or planner. They very rarely include any music or photobooks. They're usually released near the end of the year or sometimes early the next year, and relatively expensive compared to normal albums.

Single: These are generally a single track that's released digitally.
Pre-release: This is just a digital single, but specifically, a song from an upcoming album that's about to release, and released as a teaser to get people excited for the album.
Mini Album: These are usually 3-7 songs, and sometimes include the hit track from the previous album, or sometimes a prior song in a different language like Japanese. These are released instead of full albums so they can release new content more often, so you don't have to wait a full year or more for new music.
Full Album: They're less common in kpop, in order to be able to release content more often, but these are usually what normal albums are in the west, so usually 8-15+ songs. Twice released 8 mini-albums before they released their first full album, for example.
Monograph: These are premium limited edition photobook albums; see section below for more details.

Monographs are premium limited edition photobook albums.
They are in the form of a hard-cover photobook, with a pocket in the front or back cover that holds the CD.
They are all a standardized size and design, so they form a nice uniform row on a bookshelf.
They are usually released a few months after a normal album release.
At the moment, there are 18 monograph albums.
The shortest time between a normal album and its monograph version release was 2 months
The longest time between a normal album and its monograph version release was 1 year and 1 month
The average time between a normal album and its monograph version release is 5 months

Digipack: This is just eco-friendly packaging for CDs (and DVDs). Instead of a plastic jewel case for the CD, these are cardboard, and usually use a cardboard pocket for the CD or a foam cylinder that the cd is pressed onto. These are usually the same size as a plastic jewel case, but thinner, so they're more minimal and don't include as many inclusions.
Nemo: These are very small albums that don't include a CD or DVD, but instead, include either a QR code or plastic NFC puck that you scan with your phone to get the digital version of the song. Sizes vary, but they're often around the size of a deck of playing cards.