Discussing the different types of masks in the Korean Beauty World is a serious thing. One can easily get lost in this world of masking because there are just many of them.
To alleviate some of your confusions, and to answer your how's and why's, let's discuss each kind of mask.
SHEET MASKS: these by far are the most famous masks. They are usually made of soft fibers like 100% cotton or the higher end kinds, the hydrogels, with eyes and mouth holes in them to accommodate the shape of your face. The mask is soaked in tons of essences that deliver your skin lots of moisture with other side benefits like skin firming, soothing, brightening etc.