According to the Harry Potter Wiki Butterbeer is:
Butterbeer is a popular wizarding beverage described as tasting "a little bit like less-sickly butterscotch."You can read more about it HERE.
It has a very slight alcohol content, which could get house-elves fully drunk, but seemed to have a less pronounced effect on humans.
Butterbeer is served at Universal Studios: Islands of Adventure theme park and that's where Plumey bought 4 glasses of it just for the mugs:
You can make your own Butterbeer at home!
THIS SITE features this recipe:
Makes 6 servings
FOR THE BUTTERBEER
6 12-ounce bottles cream soda [I prefer IBC (which is vegan) or Polar Classics Vanilla, but any will work perfectly], substitute diet cream soda for a still yummy sugar-free version
3 teaspoons imitation butter, or clarified butter
OPTIONAL – 1 ounce light rum, per serving, for adults only
FOR THE FOAM
2 cups heavy cream
6 tablespoons sugar, or splenda for sugar free version
2 teaspoons vanilla extract
1 1/2 teaspoons imitation butter
To make the butterbeer: Set out 6 16-ounce glasses. Place 1/2 teaspoon of imitation butter in each glass. Pour 12 ounces of butterbeer into each glass. Lightly stir, if necessary.
To make the foam: In a large bowl, or the bowl of a standing mixer, whip the heavy cream on medium high speed for 3-4 minutes until it starts to thicken. Add the sugar and continue whipping until very soft peaks begin to form, another 3-4 minutes (if you need to whip more or less, then be sure to do so, the times can vary quite a bit based on environment). Stir in the vanilla and imitation butter, then whip for another 30 seconds or so, until soft peaks form.
Spoon a generous portion of foam on top of each glass of butterbeer, mixing gently, if desired (or spoon the foam into the bottom of the glass and pour the butterbeer over the top). Serve immediately.