What is the difference between a BAPI,Idoc and an RFC?

What is the difference between a BAPI and an RFC?
BAPI stands for Business Application Programming Interface. It is a library of functions that are released to the public as an interface into an existing SAP system from an external system.
RFC is the protocol used to call functions in an R/3 system by a caller external to R/3 or to call programs external to R/3 from an R/3 system.
Functions can only be called via RFC, if they are tagged as RFC functions in the SAP development workbench. They are then called RFC function modules. BAPIs are complete sets of (BAPI) function modules that model a business application.When you are familiar with web developments: RFC can be compared to HTTP and BAPIs are CGI applications.
In other words: A BAPI function is a function module that can be called remotely using the RFC technology.
BAPIs, RFCs and IDOCs are often confused. This section will help you to understand how RFCs relate to BAPIs.
BAPI is a business object. An RFC is functional code. IDOCs OR intermediate documents are standard data structures for electronic data interchange (EDI) between application programs. IDocs are used for asynchronous transactions: each IDoc generated exists as a self-contained text file that can then be transmitted to the requesting workstation without connecting to the central database.
IDoc
IDoc (for intermediate document) is a standard data structure for electronic data interchange (EDI) between application programs written for the popular SAP business system or between an SAP application and an external program. IDocs serve as the vehicle for data transfer in SAP's Application Link Enabling (ALE) system. IDocs are used for asynchronous transactions: each IDoc generated exists as a self-contained text file that can then be transmitted to the requesting workstation without connecting to the central database. Another SAP mechanism, the Business Application Programming Interface (BAPI) is used for synchronous transactions.
A large enterprise's networked computing environment is likely to connect many geographically distributed computers to the main database. These computers are likely to use different hardware and/or operating system platforms. An IDoc encapsulates data so that it can be exchanged between different systems without conversion from one format to another.
IDoc types define different categories of data, such as purchase orders or invoices, which may then be broken down into more specific categories called message types. Greater specificity means that an IDoc type is capable of storing only the data required for a particular transaction, which increases efficiency and decreases resource demands.
An IDoc can be generated at any point in a transaction process. For example, during a shipping transaction process, an IDoc may be generated that includes the data fields required to print a shipping manifest. After a user performs an SAP transaction, one or more IDocs are generated in the sending database and passed to the ALE communication layer. The communication layer performs a Remote Function Call (RFC), using the port definition and RFC destination specified by the customer model. The IDoc is transmitted to the receiver, which may be an R/3, R/2, or some external system.
Remote Function Call
RFC is also an abbreviation for Request for Comments.
A Remote Function Call (RFC) is an application program interface to R/3 applications from SAP, the German company that sells a coordinated set of applications and databases to Fortune 1000 companies. SAP customers who wish to write other applications that communicate with R/3 applications and databases can use the RFC interface to do so.