explain diff between metal core and dual shielding and flux

For the longest time I always thought there was either flux core or solid wire.

Now I learn of terms such as metal core, and dual shield.

Is dual shield using a flux core with a shielded gas?  If so why? 
Is metal core just another name for solid wire? or just the oppostite of flux core with the flux on the outside.

Wow just when you think your starting to know your stuff you get humbled pretty quick.

For those that respond dont feel like you have to write a dissertation.....I know these are broad questions.

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