DCSS is actually based on Linley's Dungeon Crawl (first released 1997). When the original dev team for Dungeon Crawl sort of went inactive, another group revived the project as Dungeon Crawl Stone Soup. Basically, it's the most updated version.
It's a fun game, and the wide variety of races & classes is interesting, but I've played (and beaten) Nethack, and I have to say that I'm more fond of Nethack than DCSS. The main problem is that DCSS's game balance is badly designed. On level 2 or 3, it's not unusual for people to encounter unique enemies (like the dreaded Sigmund), who will absolutely wreck you if you fight them at a low level, even if you know what you're doing. You simply won't be strong enough to deal with them if you happen to meet one, and will almost always be forced to run down to a lower level ASAP.
Nethack on the other hand scales enemies to the lowest dungeon level you've reached for the majority of the game. While it is possible to encounter nasty enemies on somewhat early levels (soldier ants are probably the deadliest early-game enemies), for the most part, deaths in Nethack are avoidable. If you die, you can think of ways you could have avoided it, and can improve your game. DCSS relies a bit too heavily on luck in this regard - there are some fights early on that you simply won't be able to deal with. Part of the problem stems from the fact that you don't have nearly as many tools as in Nethack. Nethack gives you a pet at the start of the game to help make the early game much less painful, there's always the Elbereth option which can be made very reliable, and you can often use tools in a variety of ways (you can avoid enemies by getting telepathy quickly, or by simply digging at any time, etc).
DCSS on the other hand seems more focused on raw combat - it's more down to raw strength or who can kill whom first. Finding a magical weapon and upgrading your combat skills via training quickly is extremely important as most of the game will involve bashing things. Nethack on the other hand, while being able to kill stuff is quite important, makes life a bit easier by providing more non-combat abilities and much more you can do both with your items and environment. You'll be relying less on finding that awesome magical sword and more on the various mundane items you can find. "The devteam thinks of everything" saying fits Nethack quite nicely.
On the other hand, DCSS is a bit simpler, so it's probably a better roguelike to start with if you're new to roguelikes since there's less to learn. You don't need to spend as much time experimenting with items to figure out some neat (and sometimes obscure) stuff you can do as it's more a bit more simplified - get lucky enough to find a poisoned magical weapon early on and you can stab your way fairly far in the game.
