I review a lot of face mask sheets on this blog, I am a skincare junkie to the core, so when I received this question, I had a face-palm moment. I always just added a picture of the mask in its pack for reviews and hauls and sometimes even showed the back of the mask pack but never really showed the masksheet. Of course before coming to Korea I did not even know about masksheets, let alone using one (or knowing how they look like), so I thought I will make this answer easy to access and feature it as a separate post on this blog!
So here is how most mask sheets look like:

* I have showed a Mamonde (Korean brand) masksheet here.
Most masksheets come in this style but some have bigger or smaller cutouts for eye sockets, nose and lips depending on the country of origin of the brand. Most Korean (and Japanese) masksheets have smaller cutouts than western brands simply because of the almond-shaped eyes of the people and other distinct features.
In addition to face mask sheets, I have also seen neck masks, masks to tighten jawline (it rests at the ears) etc. in Korea and Etude House recently launched masks for specific parts of the face – forehead, chin, cheeks, neck. There are plenty of eye patches, nose patches, chin patches and lip masks in the market now!
I have tested and reviewed face masks from various Korean brands – The Face Shop, Etude House, Innisfree, Mamonde, Holika Holika, It’s Skin, Skin79 etc. and you can find them on the blog easily by searching in the search box in the sidebar. Hope this post was helpful to you!