Interior and exterior routing protocol differ in how they route. Interior routing protocol are designed to be using inside and AS (Autonomous System) while Exterior routing protocols are designed to be used between AS.
Here is the definition from the Exam Guide:
IGP: A routing protocol that was designed and intended for use inside a single autonomous system (AS)
EGP: A routing protocol that was designed and intended for use between different autonomous systems
There is only one EGP (BGP) but many IGPs.