A domain name is online address through which a web users can access your website.
Yes you can use your domain name for email, you need purchase web hosting for email hosting to use your domain for email.
Domain name is your online identity, you can use domain name to host your website, you can use it for email, you can use domain to point to your facebook page or you can use your domain to point it to your business listing in third party portal like on amazon or flipkart.
Domains are registered instantly after payment.
Yes you can sell or transfer domain name, domain name is your property.