How do Escrow (especially Bitcoin) services work?

It doesn't make sense.

For example: You send your coins into a secure Escrow service, the seller sends what exactly into his escrow service?

That's the part I don't understand. Can't the seller just sell a dummy product or whatnot?

I am new to Escrow so please explain this to me.
Update: How safe are Bitcoin Escrows? Walk me through the process
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