Every Christmas tree stand is made to hold a tree straight and keep its water in, and few are much to look at, so something goes around it. A skirt is a circle of fabric laid flat under the branches. A collar is a rigid ring, of rattan, metal, or wood, that stands around the stand.
They hide the same thing and leave the floor around the tree quite different. The choice comes down to how much floor the tree can have, what goes under it, and, for a cut tree, how easily the water can be reached.
A Skirt Spreads Under the Branches

A skirt lies flat and reaches out to the branch tips or a little past them. It covers the floor where needles fall and catches them, which matters most under a cut tree, and it gives presents a soft place to sit. Under a tree with a lot of presents, a skirt is the one that still shows between them.
It also takes more floor. A skirt sized to a full tree spreads well beyond the trunk, and in a small room that circle of fabric can reach the edge of a rug or the foot of a chair.
A Collar Rings the Stand

A collar sits close around the stand, so the floor beyond it stays clear. A tree in a collar can stand nearer a sofa or a doorway, and the tree can look less like it has taken over the room. A collar also lifts the tree visually, giving it a firm base like a planter.
A collar doesn’t catch needles or give presents a surface, so they sit on the floor around it. A rigid collar holds its shape where a skirt gets pushed into folds by feet and pets, which suits a busy room.
Measuring for a Collar

A collar has to fit around the stand and under the branches. Measure across the stand at its widest point, usually from leg tip to leg tip, and choose a collar whose inside measurement at the base is wider than that. Then measure from the floor to the lowest branches, and choose a collar shorter than that, so its top sits just below them rather than pushing the branches up.
A collar made in two halves, or one with a hinge, fits around a tree that is already standing. A one-piece ring goes down first, with the stand and the tree set inside it.
Measuring for a Skirt
A skirt is sized to the branches, not the stand. Measure the spread of the lowest branches across the tree and choose a skirt about that wide or a little wider, so it reaches the tips. For a full 7 foot tree, that is a much wider circle than for a slim tree of the same height.
Skirts open with a slit to the center, closed with ties, buttons, or a hook strip. Lay it with the slit at the back of the tree, where it won’t be seen.
Watering a Cut Tree Through Either One

A cut tree needs its water checked every day, and both a skirt and a collar sit over the stand. A skirt folds back on one side to reach the bowl. With a collar, there has to be room to reach over its rim or a gap to pour through, and a funnel with a long spout helps reach the bowl without lifting the collar off.
An artificial tree needs no water, so either one works the same all season.