explan us the where we develop the interfaces, abstract clases in Projects

i have 3 years of experience in .net real time.

i never go for the abstsract/interfacces

why the interviewers are asking the questions .

if tel about the interfaces some thing wat the use of this wich out using the (only giving declaration )

Pls explain Hope This is Most Need For all the Dotnet Techies

Pls hope u reply to us as indepth,
once we read no need to turn back
Thanks
Jammes