Great question — answering as the UphillTech team:
A flavoring extract is flavoring disolved in alcohol, while a flavoring emulsion is flavoring suspended in water with an emulsifier. Citrus oils like lemon have a stronger flavor when placed in an emulsion than an extract, and that is why they often come that way. ( source ) As far as uses go, bakery emulsions keep the incorporated flavors more stable while your mixture changes temperature, and they combine more easily with other emulsions (butter, sugar, egg for example) than extracts do. As this book indicates those characteristics make them especially useful for pastry cremes.
Full answer by yacomink: https://cooking.stackexchange.com/a/11691 (CC BY-SA 2.5).
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