Nokia provided you with good info, here is some more info for you to read

Infrastructure Master - This is another domain-specific role and its purpose is to ensure that cross-domain object references are correctly handled. For example, if you add a user from one domain to a security group from a different domain, the Infrastructure Master makes sure this is done properly. As you can guess however, if your Active Directory deployment has only a single domain, then the Infrastructure Master role does no work at all, and even in a multi-domain environment it is rarely used except when complex user administration tasks are performed, so the machine holding this role doesn't need to have much horsepower at all.
From HERE

And some good info on FSMO roles HERE

.C.