what is domain-on-hold and domain deleted?

Today I came upon one site which sells "new domain", "domain-on-hold", and "deleted domains".
I couldn't understand what they mean by "domain-on-hold" and "deleted domains". Does anyone know about it?

I guess "domain-on-hold" is the grace period given by domain registrars to their customers to give them an opportunity to renew their expired domains.
Now, can anyone help me?

I think deleted domain names is those domains, for which the users have not renewed the period. Suppose a domain name is very popular, and if the domain name has been deleted by the service provider, then anyone can use that domain name and established his business without much effort.