Sour drinks are favorites for many and within this family there are two common variations, the Sour and the Collins. They're all very similar and related so let's break them down.
Collins and Sours are base spirit plus sour (either simple syrup with lemon or a pre-mixed sour), when making a Collins switch to a tall glass and top with soda. For example you have a Whiskey Sour, which you'll serve in a sour glass (or other short glass), and if were to pour that into a collins glass over ice and top it with soda you will have a John Collins.
This easy memorization trick of association knocks out the majority of the most common sour cocktails and can be applied to all of the base spirits. Tom Collins and Gin Sour both have gin; Vodka Collins and Vodka Sour, vodka; Juan Collins and Tequila Sour, tequila, etc.
Sours are expansive and the list nearly endless. Common variations include the Absinthe Sour, Amaretto Sour, Apricot Sour, Gin Sour, Grand Marnier Sour, Kahlua Sour, Midori Sour, Rum Sour, Scotch Sour, Tequila Sour and Vodka Sour. The Pisco Sour uses pisco but also adds egg and the Frisco Sour adds Benedictine to the Whiskey Sour.
Two of the Collins drinks do not spell out the spirit but I have another association trick that keeps John and Tom straight in my head. Tom Collins is made with gin because it was originally made with Old Tom Gin and when it comes to the John Collins I think of "Big Bad John" (whiskey's manly perception) and his many friends like Jack and Jim.