Hello rakesh,
Before starting replying to your very basic question, I think it's right for me to refer this thread to go through if you haven't already The SCN Rules of Engagement.
As I said this is a very basic IT query (I chose IT instead of SAP), and any attempt to reply would be a big exercise, but be only giving you an outline understanding. You would get complete understanding gradually after vigorous exposure in your field over a period of time.
Have you heard of RDBMS (Relational Database Management Systems)? Search Google and have concepts. Any Software deals with Databases. Database is nothing but storage place of user's information. This Database Mgmt will have several tables which hold this day-to-day records (transaction data) in the form of Rows and Columns. When a PM user creates a Notification this generates a row in table QMEL. Similalry if it is Order this generates a Row in table AUFK. Similarly every module has 100s of tables and all these tables are connected across modules which makes the software an Integrated one.
In the tables few tables are One time entry tables like your Equipment information (EQUI) during implementation. (After that it is very rarely handles whenever a new equipment comes) Similar table is Material Table (MARA) in MM module. After implementation whenever a job is done on an Equipment (listed in EQUI table), user records it in system which adds a row in table QMEL and/or AUFK. Similarly when user does Purchase action of a material (listed in MARA table) the tables EBAN, EKPO, EKKO are updated (rows added).
You have standard reports for both Masterdata and Transaction data in the system, which retrieve information from these tables and give you with the help of ABAP programs. If you do not have the required report then ABAP person makes it using these tables.
See this picture to understand how user connects tables of different modules to generate his report which is not available in standard.
See the KEY symbols in tables. Every table has one or more Primary keys which makes a unique identify of the Row.
Also see a tables picture of PM module posted in this thread: Plant Maintenance Tables
About Interface, I feel It is a bit distant topic at this stage, which involves XI (Exchange Infrastructure) people.
Good luck
KJogeswaraRao