Think of it this way (as we were taught to in the Society for Creative Anachronism, when making our medieval garb): A tunic is basically a T-shirt. From there it's only a matter of what length it is, what fabrics and trims are used, what kind of sleeves it has, and how it's cut.
Strictly speaking, a skirt is a garment whose top is at the waist, not the upper body, and skirts can range from very short to floor-length (or longer).
A tunic covers the upper part of the body like a T-shirt does, and the length varies. It may or may not have a belt at the waist.
Whether the bottom is flared or not doesn't matter.