AS (Autonomous System) is group of networks controlled by one administrator (With common administration), as one ISP or enterprise organisation. According to way how they exchange routes, within an AS or between them, we can divide routing protocols to IGP and EGP
Interior Gateway Protocols (IGP) are used within an AS (organisation). Their scalability has limits. most known protocols are Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF) and Routing Information Protocol (RIP). Other common name for this group of protocols is Intermediate System to Intermediate System (IS-IS).
Exterior Gateway Protocols (EGP) are used between different AS. At this time, only protocol in use is Border Gateway Protocol (BGP).