Inter and Intra are both prefixes. They sound similar and can often be confused for one another, but have two different meanings. Inter is between or among different things. Inter means between two things. Examples of interbeing used are words such as interschool or interuniversity, which would mean between two schools or between two universities.
The prefix infra means inside or within something. There are many examples of this prefix being used. For example, an intranet is normally referring to a network system that is used within a company. Intracranial means within the skull, and intracompany is within a company.