Why is it that some chess engines have the word deep in their names? It does seem to be a common trend, but not one that is universal.
It used to be the case that the "Deep" specified the multiprocessor version. At least for Fritz, Shredder and so on, the "old" engines. But I guess nowadays every engine is a multi processor engine …
This naming trend was surely started by Deep Thought, the first seriously strong IBM chess machine. And of course Deep Thought was named after the famous computer from the hitchhiker's guide to the galaxy.
"Deep" is simply just a marketing technique. It's a tactic to sell you more, and sell you at a higher price.
Consider this, "Fritz" and "Deep Fritz". What does that mean to an average user? "Fritz" might sell for $50, "Deep Fritz" might sell for $100. What would an average user with little chess engine knowledge think?
They might think like this: "Oh, it's a DEEP version of the software, so it must be GOOD because DEEP Blue has beaten the World Champion. This software must be DEEP. Why not pay $50 more for a world-champion-level product?".
Chessbase is trying to paint a mental picture on you:
"DEEP" Fritz == "DEEP" Blue > Kasparov == World Champion > the $50 difference.
Technically, Chessbase explains the difference is due to multi-threading. But this is just an excuse. They simply make the "normal" version single-threaded. It's a 10s work. Everyone can do it.
This is really a very standard marketing technique. What do you think when someone says "I have a new porsche car"? Probably something like, "rich", "successful" etc.
Then, why they're also selling the "normal version if they want to make more money? That's because a consumer needs a reference, they need something to compare to justify that they should buy the more expensive version. Say, a small coke at McDonald was $3.5 and a large coke costed you $4.0. Only a 50 cents gap, lots of people would get the large coke because it's relatively cheap to smaller version of the same product.
It's all about marketing. Chessbase (and also Shredder) is here to make profits. They're trying to relate their product to something that you're familiar with, to make more money.
It has to do with the way the chess engine calculates the its moves. It constructs a partial Game Tree and then runs a search algorithm to calculate the best move. The deeper the search algorithm goes (that is, the deeper are the "roots" of the partial tree), the better the move will be.