What type of db is stored in app_data folder of asp.net project

few questions about local db and sql express db.

1) i found many time a db with .mdf extension in app_data folder. that db is local db or sql express db?

2) is there any process which help me to determine that db type in app_data folder ?

3) what is the difference between local db and sql express db? or both is same ?

4) what is sql compact db and how it is different from local db and sql express db?

please answer point wise. thanks