Plants and trees are either sold as pot plants, plugs or bare roots. What type is used for a specific product is indicated on the detail page of that specific product.
People are most familiar with Pot plants, these can be shipped almost year round, and we use pots varying from 7cm thru to 25cm diameter for most of our Perennials and Shrubs.
We also use 5/6cm plug plants for a lot of our summer plants. Plug plants are smaller than pots, but have just as much growing power. They are more effective for sending in larger quantities, because they contain less Soil.
Bare root is used for some of our larger Fruit trees, Hedges and some Perennials (peonies). These are usually strong growers that have been grown in full ground, not being limited to a pot and they usually adapt quicker to your garden than pot grown plants. However pot plants can be planted year round, for bare root this is not the case.
There is also a large group of bulbs (Tulips etc) and tubers (Dahlia's etc) that everybody is familiar with. bulbs are specified by their circumference in cm, tubers are mentioned as Size I, II or III which is an indication for their weight (I being heaviest). Bigger does not always mean more beautiful, some of the nicest Tulip Varieties are the smaller ones, but sometimes size matters.