domain name system - What's the difference between FQDNs and Hostnames

First of all, I wanna say that I know how DNS Servers work and also I know about the records in a DNS Zone File!

I'm not an expert but I have some knowledge about them. I'm confused! why?! because I don't know what's the exact difference between hostnames and FQDNs.

I know that some thing like my.example.com. is a FQDN. somebody told me it's a FQDN because of the existance of "dot" at the end of it.

I think the final dot at the end of it reffers to the root dns servers in the world, right?

Is my.example.com a hostname? (note that it's doesn't have any final dots!)

maybe "my" is the hostname?!

Actually i'm confused!

What's the difference between a hostname and a FQDN, guys?!

Sorry, My english isn't very good. excuse me if I made a mistake.

Thanks in advance.