More than just trees are available as bare root in the spring. You will find--asparagus, rhubarb and lots of shrubs. Nurseries get bare root stock in the early spring, then what is not sold will be planted and and sold in pots, when rooted out, usually in Mid Summer.
There are basically three ways you can buy nursery stock. Most people know about containers, fewer know about bare root and balled & burlap. It sounds like it would make a good novel or movie. So with out further interruption here is the scoop on Bare root-, Balled & Burlap and Containers.
A Little Bit About Bare Root
Depending on where your live, you should be able to find B/R plants between February and March. B/R is a dormant plant which is not planted in soil. This is the least inexpensive way to purchase plants. Not all plants come bare root. Some shrubs do and most deciduous trees(trees that lose their leaves--go dormant). You will not find evergreens in bare root. The roots of bare root plants must be kept moist and in some kind of mulch before planting. Do not let the roots dry out.
If you buy B/R at a nursery, they should do all the necessary pruning of the roots and branches. Make sure there are not any broken roots.If you have more than three feet of branch growth with out any side limbs--prune it.Plant as soon as possible. Dig your hole a little larger than the roots on your B/R. Add about 1/3 to 1/2 organic matter to the soil you have taken out. Fill the hole up with water, then let it drain.Most B/R trees will be grafted. ( a little bump a few inches up the trunk, above the roots). DO NOT plant above this--YOU WILL KILL THE TREE--KEEP THE SOIL OFF OF IT. OKAY, I'll calm down. Anyway--you should see where the soil line is on the trunk.Any soil you have left over you can use, by building a little water basin around the tree to hold in water. After planting your B/R, stake the tree up for the first year of it's life. I prefer to fertilize a couple of weeks after planting (less chance of burning the roots).Some people mix fertilizer into the soil when they plant--your choice.
Balled and Burlap
B&B is a way to buy larger shrubs and trees in the early spring, for less than what you would pay for container grown material. B&B are plants that have been dug up and have kept the dirt intact with the roots. You can buy evergreens this way. Once the roots start growing in the spring they need to be planted, so you will only find B&B in early spring. (February-March). Just like B/R--B&B can save you money. When you purchase your plant it is important not to drop it. The plant could go in to shock. Dig your hole a little larger than the B&B--add some organic matter--water and mix--Place the B&B in the hole--Place ground level with the soil line of the plant. Cut the twine that is wrapped around the trunk--remove if possible--if not, tuck down into the hole--do the same with the burlap--unwrap around the trunk and cut some of the excess off. Tuck the rest into the hole, it will break down. Fill in the soil, water real well and build a basin around the tree.Remember to give your tree or shrub room to grow---Have fun.