There are three primary types of mergers, from the standpoint of industry structure. These are:

Horizontal Mergers: Horizontal mergers are the most likely to be challenged as reducing competition. They involve the merger or combination of two firms in exactly the same industry. A merger between MCI and Sprint long distance carriers would be a horizontal merger.
Vertical Mergers: Vertical mergers are the merging of two firms in the same industry but at different points along the production process. A merger between a fabric milling company and a clothing manufacturer would be a vertical merger.
Conglomerate Mergers: Conglomerate mergers are mergers between two firms in mostly unrelated industries and are the least likely to be challenged. A merger between a tobacco company and a brewery would be a conglomerate merger.