Here's where things get a little tricky. Some bartenders will make a Tom Collins with bar mix (or sour mix) because the simple syrup and lemon are right there in one bottle. Other bartenders will pick apart the sour mix and use simple syrup with fresh squeezed lemon juice separately. Which is better? My humble opinion always pines for the fresh route, but either makes a decent drink.
Then we get into the other "Collins" drinks. There are many fancy concoctions that build off the collins base and you can include all kinds of fruits, berries, liqueurs or anything else you want to customize it (ie. Jazz Collins, Blueberry Collins, etc.). The most popular ones, however, are the Tom Collins (gin), the John Collins (bourbon) and the Vodka Collins (obviously vodka). When I was first learning the difference between the first two I had to create a trigger to remember which had which base liquor: for the Tom Collins think of Old Tom Gin and for the John Collins I think of the old country song that's on a 45 in my jukebox, Big Bad John (by Jimmy Dean), because that makes me think of backwoods bourbon stills. That's my association trick to keeping the two drinks straight, but I'm sure everyone has their own.