"D&D"- Simply put, any game system that anybody sits down to play in dungeons and encounter dragons that is Trademarked by a major game publishing using "D&D" in the title. Words like "white box" and 5E mean something in conversations about this category.
"Old School"- This includes a lot of D&D products, but also has ones like Tunnels & Trolls, Call of Cthulhu, and say Empire of the Petal Throne. Basically, if I or another person that has to worry about high blood pressure because of our age played this as 11-14 y/os, it's "Old School." I've grown old enough to watch "new wave" games like Vampire:the Marketing and Shadowrun join this category. Palladium Publishing is in this group, no the curmudgeons do not get their own listing.
"Big Press No Play"-Mostly because of the advent of Kickstarter and other interweb panhandling these are publishers that keep your storage bins full of whatever they are selling every couple of years. This category has included what would become very successful publishing houses and moved into "Old School" territory. It also includes anybody that has the license for any TV show released in the last two decades. To quote someone that likes to avoid being mentioned by name on-line, "Yet another rules system that will have handfuls of players for thousands of hard-bound books."
"OSG/OSR"- Games which try to get back to the days of yore when role-playing was dungeon-crawling through beautifully crafted maps with rules systems that feel familiar to the "Old School" RPGer even if it never was really like it. This can include any variation of a TSR game including Gangbusters, Gamma World, Boot Hill, Top Secret, and may include the new editions of those games from the refurbished TSR company. Not always but most popular is the use of a 20-sided die and a few charts. It can use game mechanics with ridiculous varying amounts of number ranges because of unconventional dice. Other times it focuses on the approach of the GM towards their player's Characters lifespan-- the shorter and more pointless a PC is, the better.
"Indie"- Always dear to my heart because "I haven't played D&D since 1980" (a lie that is almost true). Many try for new markets, like people younger than 50, but failing that to video games, go for niche markets. I don't just mean books for parents to
"BX"- A new one to me. Looking at the authors I see using this, I'd say it means using the concepts of "Basic" and "Expert" as the follow-ups to AD&D, but for the crowd buying the OSG/OSR products more than the official D&D brands. I think the "X" includes advanced directions for wargamers wanting to LARP with their miniatures.
"Rules Lite"- Oh how I love these goofs. On the extreme, if it's over a page, the author needs to revise. More often than not, ten pages of typing is seen as an encyclopedic tome meant for your Grandmother's Wraith/Warhammer 40K crossover campaign. When these people go setting, they deconstruct with the best of any RPG writer. The guys from QAG (are they still around?) comes to mind-- One author did like the entire Cold War in two pages. Ken St Andre, author of T&T, the first "not to heavy on the rules" RPG game* ever, is trying his own hand at it with his latest release Fours.
And now my newest category! To repeat the quote where I heard it. "...However on G+ someone mentioned that to them a game was not rules light because it used UnfamiliarTerminology and Mechanics..." and "Gee" because like game, or G+, or something.
"U-TaM-Gee"- Any game for the reader that has not read a lot of games before.
*RPG game- a Fantasy Adventure Game entitled Role Playing Game but actually meant to be used as a basis for a collectable card games based off of Warhammer: Tolkien Edition.