This is what you get when you make a gin sour and turn it into a highball. It's a fine summer refresher when it's not too sweet. For that reason, it's best to use fresh lemon juice rather than the bottle Collins Mix available from some soda bottlers.
Who was Tom Collins? Nobody and everybody. In the mid-to-late 1800s in New York City, there was a practical joke that became all the rage. On the crowded streets of The Big Apple, a prankster would approach someone, even a stranger who had a vague look of recognition, and in a confidential tone say something along the lines of "You really have to do something to protect your reputation against those terrible things that Tom Collins is saying..." Since there was a good chance that the victim actually knew someone named Tom Collins, the joke was to watch the victim go striding off in righteous wrath.
Tom Collins had a somewhat popular brother John, made with whiskey. There are siblings in some books, but they seem to have been made up out of a sense of completing the catalog and never caught on.