使用mdf vs sql db server有什么优点/缺点

What is the difference between using SQL Express .mdf file and an actual SQL Server DB? Why wouldn't I use .mdf file for everything? Is there a size or performance limitation that would prompt me to use SQL Server DB? I currently use actual SQL Server DB's for every project I work on so I am just curious to know if an .mdf file might be more cost-effective.What is the difference between using SQL Expres