Difference between Create login, credential and user in sqlserver
Login: 
A login is any principal that is granted access to a SQL Server instance.  The access can be granted to domain users, domain group, or SQL Server authenticated accounts.

Credential:
 A credential provides a mechanism to store login information for a domain account within SQL Server.  The credential can then be used to pass that authentication information from into another login or a proxy to grant it permissions to resources external to SQL Server.

.....use of Login
if you need to login and access a SQL Server instance, you’ll just use a login.  Regardless of your needs, if you need to get in and do work, the login is through route to go.  The other two won’t help you out at all.

Use of credentials..
Credentials get brought in when permissions from domain users need to be granted to accounts or services that wouldn’t usually have those permissions.  For instance, if an assembly required EXTERNAL_ACCESS, those permissions could be granted through the use of a credential.  Credentials can also be used to grant SQL Authentication accounts access to external resources.