What is the difference between the Global Catalog and the Infrastructure Master?

The Global Catalog server maintains a partial, read-only copy of every domain in a forest, and is used for universal group storage and logon processing, among other things.

The Infrastructure Master is a Flexible, Single-Master Operations role-holder in each Active Directory domain that maintains internal references to objects that reside in other domains. See this article by fellow TechTarget expert Dean Wells for an in-depth look at the relationship between these two Active Directory components.