in general, a multiple domain account has one login for all the domains. (same login/password) log in once, and can see and change all the files under all the domains. this is handy for one person who owns several domains.
a reseller account has the ability to create independent domain accounts, with unique logins/passwords. this is useful for giving accounts to other people. and the "other people" cannot see or change each other's files.
If you're planning on reselling your account space then you should go for a reseller account. People who signup for reseller accounts who do not intend to resell the space usually have a larger number of domains for their business and wish to have easy control of them under one account - you do not normally find multiple domain accounts which offer more than 10 domains, this is why people sometimes opt for a reseller account. There's no difference really other than what CD Burnt said about independant domain accounts.
In the start there could you host only one domain on a shared hosting and multiple on reseller. That was two different things. Now, many people and companies have more domain names. So there are multiple domain shared hostings. It was and somewhere still is marketing ($) idea.
To add to what CD Burnt said, Reseller Accounts are also useful to web designers and people who are hosting more than one website as well. It allows them to administer their domains much more effeciently than having a shared hosting account with a whole bunch of addon domains.
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Reseller accounts now offer a large number of hostable domains, if not unlimited domains.
Multiple domain accounts are geared toward smaller individuals with the need for several online presences without the business aspect or large limits.
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