Nursery ordering them to pot up and sale...here are some examples here that may help you understand them better.

Here is a link that might be useful: plugs

Simply put, in the business, plugs are generally those plants in very small sizes one usually grows by seed. Only you don't have to sow or plant the seeds. They are already sown into trays and grown on enough that the roots fill the little cells. There will be anywhere up to five hundred little square cells of these plants to a tray or as few as 125. The larger the plug, the less time it takes to grow them on to size. Examples of material you might find in plug form are seed grown strains of petunia, marigolds, tomato plants......many annuals not patented.

Liner stock is how one often describes those plants grown from vegetative cuttings and rooted up in little cells. That encompasses the majority of patented annuals and perennials or even some nursery stock, including woody. They'll come in seperate cells as well, and are often larger than plug material and definately more expensive.

I use mostly liner stock for things like hanging baskets and fancy combination planters. I also order them in the earliest of my plant material. The plugs come in next and often just 6 weeks before I expect to have them ready for sale.