According to me, Domain masking is actually hiding your original domain name with any other domain name. This can be done by redirecting a domain name to any other website or having 2 domain names for a same webpage.
Parked domains are the domains which are not used. These domains do not have any email accounts, webpage etc. like a domain name has. These domains are just taken and kept either for development or for selling them in future. This is generally done by resellers as they can sell domains if they have them already registered.
hi mat, in addition to caroline
if you have alot of domain, people usually park it to domain parking company that give some partial revenue for domain owner.
I have parked domains and they have email accounts etc. I use them if i have more than one tld for the same website. An example would be having mysite.com and mysite.co.uk, I would park one on top of the other as they are the same site.
As to your question as to what is the difference between parking and forwarding, probably support can answer that question better than i ever could, but am sure they both achieve the same result, just different ways of redirecting
Forwarding from my understanding will just redirect from one domain to the other, parking allows you to use both domains without it changing in the URL bar