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.