MySQL database datafiles in windows SystemData directory

Question

When I create a database and some tables in MySQL Server, it makes some data files in Windows SystemData directory in C:\ProgramData\MySQL\MySQL Server 5.5\data\mydb\.

Each table has three files with these suffixes:

TableName.frm
TableName.MYD 
TableName.MYI

I don't know what those exactly are.

Please explain to me what those are and what is usages?

Thanks in advance.