Tying is the anti-competitive practice of requiring de facto or de jure the customer to purchase a certain package of goods together. It is implied in this that one or more components of the package are sold individually by other businesses as their primary product, and thereby this packaging would hurt their business. It is also implied that the company doing this packaging has a significantly large market share so that it would hurt the other companies who sell only single components.
Horizontal tying is the practice of requiring customers to pay for an unrelated product or service together with the desired one. For example all Acme woodburners come with Acme iceskates.
Vertical tying is the practice of requiring customers to purchase related products or services from the same company. For example, an Acme automobile only runs on Acme gas and can only be serviced by Acme dealers.
Tying need not be done by a single company; companies can conspire to enage in this practice.