What is the difference between EMI filters and normal filters?

 

Nothing.
A EMI is just a specialized type of a "normal" filter that is designed to suppress EMI frequencies.

 

what is its frequency ?

 

Hello,

The following page of the EDUCYPEDIA has a lot of links with information on EMC and EMI:
http://educypedia.karadimov.info/electronics/emc-emi.htm

Bertus

 

what is its frequency ?

There is no one frequency.
Generally they are a low-pass type that filters all frequencies, except possibly for any of interest that may need to pass through.
Their design depends upon the application and the degree of EMI encountered.

 

Thank you so much