What's the Difference?

My friend had a dish of these on her kitchen counter when I visited her this morning.
'Ooh, Smarties', I said.
'They're not Smarties, they're chocolate M&Ms', she said? 'What's the difference?', I asked.
'Erm, well these have little Ms printed on them.'
And so ensued a discussion about what makes one product distinct enough from another to allow it to be sold by another company without being sued.
All very interesting, but I still couldn't have one!