Short definition
A trained wine professional in a restaurant who manages the wine list, recommends pairings, and handles wine service.
The sommelier is your ally, not your judge. Their job is to find a wine that matches your food, your taste, and your budget — in that order. Tell them what you are eating, what you like, and what you want to spend. "Something like X but cheaper" is a perfectly fine request. The best sommeliers get excited about finding you a $40 bottle you will remember, not pushing the most expensive option.