MDF file?

Hello,

What is the purpose of putting an MDF file in the “app_data” folder?
is it a way to avoid using microsoft sql server?

currently i’m paying a lot of extra money (more than hosting fees) to my hosting company for mssq database.
can i use an mdf file like we used to do with *.mdb files and avoid paying money for mssql server hosting?

can someone please explain.

thank you.