Why login is required?
Login is basically required for the application to identify which user is making the request and accordingly, serve related data to him/her.
What is a login form?
A login form utilizes the credentials of a user, in order to authenticate their access. It generally consists of the typical username or email and password. But more fields can be added to improve the site's security.
What is login in app?
A login application is the screen asking your credentials to login to some particular application. You might have seen it when logging into facebook,twitter e.t.c.