Butter Tarts, as made in Canada, are tarts made with melted butter, brown sugar, egg, walnuts, and white vinegar. The mixture is spooned into tart shells and baked.
Some purists say that's all a Butter Tart should have in it, but in fact, that's a recent school of thought. There are firmly-established, long-seated traditions that include raisins, or currants, or walnuts, or a combination of raisins or currants and walnuts. The earliest printed versions call for currants. A few late 20th century versions add chocolate chips, horrifying all of the above schools of thoughts.
How dark the tarts are depends on how dark the brown sugar is.
Some recipes swap in lemon juice for the vinegar. Though vinegar was the original ingredient, the swap for lemons shows the evolution of lemons becoming more affordable over time. The purpose of the vinegar is to cut back a bit on the sweetness, and as it's stronger, vinegar does a better job of that than lemon juice can do.
The cooked filling may be more runny, or it may end up more solid and slightly puffed (sometimes referred to as "set.) Some versions of the filling end up with a golden sugar crust on top, some do not; it all depends on the recipe. Some runnier versions since the 1930s swap the brown sugar for corn syrup. The tarts can be deep or shallow.
Butter Tarts are particularly popular in the provinces of Ontario and Nova Scotia, though in recent years there has been encouragement to think of them as a pan-Canada dessert.
They are very closed related to American Pecan Pie (which generally adds cornstarch to the filling), and to Scottish Ecclefechan Butter Tarts. In fact, depending on the Canadian Butter Tart recipe version, and the Scottish Butter Tart recipe version, the two can be identical (some Scottish versions use mixed fruit, though historically just currants was more common.) The Scottish version is occasionally made as one large pie; Butter Tarts in Canada are not. In fact, you could really regard Butter Tarts as a "cheaper" version of Ecclefechan Tarts, that are stingier with the fruit and nuts.
Butter Tart with Raisins
© Denzil Green
Comparing Butter Tarts as made in Canada, though, with the Ecclefechan Butter Tarts of Scotland, which have been made since before Robbie Burns was born, and bearing in mind that the two Butter Tart epi-centres of Ontario and Nova Scotia were both heavy centres of Scottish immigration, and that the earliest print recipes are attributed to distinctly Scottish names, it staggers belief that an identical recipe just happened to be created out of the blue in regions where the immigrant families would have known them from back home. Heavy areas of Scottish immigration in the American South became the birthplaces of Butter Tart's kissing cousins, Pecan Pie and Pecan Tassies.
A 1909 recipe referred to them as "Currant Tarts." 
Ruplins Baking Co. in La Crosse, Wisconsin advertised something they called "Butter Tarts" as part of their Christmas 1912 lineup. 
Literature & Lore
1 cup currants
1 cup brown sugar.
1 / 2 cup salted butter, softened
2 large eggs, slightly beaten
12 tart-sized pastry shells
Put currants in medium-sized bowl and cover with boiling water for five to ten minutes. Drain currants, discard water, and place currants back in the same bowl. Whisk in brown sugar and butter, and combine well. Blend in eggs.
Spoon filling into tart shells until three-quarters full (make sure currants in the liquidity mixture are evenly distributed in each shell).
Bake in bottom third of 400F oven for about 15 to 20 minutes, or until filling is puffed and bubbly and pastry is golden. Let stand on rack for 1 minute. Immediately run flat metal spatula around tarts to loosen (this will help prevent sticking). After five minutes, carefully slide spatula under tarts and transfer to rack to cool. Makes 12. 
 Elphick, Katherine. RVH cookbook boasts one of first, printed butter tart recipes. Country Country Now Magazine. 12 February 2008. Retrieved October 2012 from http://www.cottagecountrynow.ca/cottagecountrynow/article/387501
 Selected Recipes. Ladies of Young Street Methodist Church. 1909. As mentioned by: Baird, Elizabeth. The Sweet Treat Debate. Canadian Living Food Blog. 15 December 2009. Retrieved October 2012 from http://www.canadianliving.com/blogs/food/2009/12/15/the-sweet-treat-debate/
 Advertisement in La Crosse Tribune. La Crosse, Wisconsin. 23 December 1912. Page 10.
Legue, Suzanne. Butter tarts soar from ‘bucket list’ to bestseller. Cottage Country Now Magazine. 29 April 2008. Retrieved October 2012 from http://www.cottagecountrynow.ca/cottagecountrynow/article/379975
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-- Julia Child. (15 August 1912 - 12 August 2004)