Leggings are made of thin stretchy material. They are like a compromise between pants and stockings. When your skirt/tunic thing is short enough that you might expose yourself legging as the way to go, because they are totally opaque, but are not bulky enough to change your profile like pants. Many people (particularly college undergrads?) wear them with regular length shirts on top. The objection is that leggings are not pants and I tend to agree.
I am not generally one to shame someone for their choice of clothing. Leggings can be incredibly comfortable, and you are under no obligation to put together a nice looking outfit when you leave the house. Ladies, if you want to throw on whatever is close at hand, that is fine. That is a legit way to choose your clothing. The reason leggings as pants frustrates me so much is that many of the ladies wearing them look like they took the time to put on make up and do their hair, and find coordinating accessories. If you did all that, then you though leggings and a t-shirt was a good looking combination? If so, then lets talk fashion choices a minute.
If you insist on wearing leggings as pants, I am going to analyse them as pants. They are knit fabric, elastic waist pants. Elastic waist, knit fabric pants are not cute. They have a well established tradition in our world of clothing choices. They signal something, something we all know they signal, but which apparently by calling them leggings and not elastic waist pants, we are pretending they are free of the sigma of. People do not wear elastic waist pants to be fashionable; they are pants which one wears out of necessity, even though they are not fashionable, because you are a member of a class for whom a particular function is much more important than form. One wears elastic waist pants because you need the "on off" function of pants to be easy to operate. The well established class of people who already wear elastic waist pants is people who have trouble getting real pants on and off when they need to pee: Toddlers and the elderly. Those are the people who wear elastic waist, knit fabric pants. I know that hug your firm young bottom nicely, but so will a good pair of jeans and I will not pretend that wearing toddler pants projects a pulled together and fashionable appearance.