y we use name="user_name"

When the information in the form gets sent to the server, that info gets stored in name-value pairs.

The server will get that info and store it like this:

user_first-name = "John" user_last-name="Doe"

Here's some reference http://www.w3schools.com/tags/ref_httpmethods.asp

You can set the name attribute to whatever value you want. It could be name="this_is_a_super_long_and_unnecessary_name_value" or it could be name="name" or name="email_address".

Typically people try to name things to relate to what they're needing for the form. So in this case, "user_name" would be a good choice for a field in a form that is needing a "user_name" for a login or something along those lines.