Explain ER Diagrams ?

Seriously, If you don't know what are ER diagrams than probably you need to start DBMS from start. They are one of the most basic feature of Database design and though No will ask you to design them (unless you are sitting for a DB company) during interview but still you should be very comfortable with them. Atleast know all ER symbols.
I will try to write a good detailed article on them later here :-)