It is a container which holds drinks. They are made from a polymer material that is flexible yet sturdy. They are used in situations where using glass ware is not possible. They sometimes have thermal abilities to keep cold drinks cold and hot drinks hot. Some are only appropriate for holding cool or cold beverages because they will melt under the heat of hot beverages. They are not that durable and are only meant to be used one time. They cannot be used in the microwave to heat liquids because they will melt and plastic cups cannot usually tolerate the heat associated with the dishwasher, they will melt in that environment as well.
They are generally used for social gatherings where there are many people or few people but using glassware is impractical. The impractical use of glassware may be relative to the location of the gathering, like at the beach or other outdoor venue or the sheer number of attendees at the gathering may make using glassware not a sensible option. They are also used by some night clubs that get very crowded to prevent glass from being dropped and breaking.
They have environmental indications as do the paper cups. They have larger replaced paper cups which was the first disposable option for cups. There is no clear difference between the two options. Both are recyclable yet both end up in the landfills.
Plastic cups are a great disposable option for parties and other gatherings where using glassware is just not practical they are relatively inexpensive around a few cents each when bought in packs of twenty or more.
They do wind up in the landfills and the recyclable life of them is not that long but they can be recycled.