The Dungeon and its branches are full of different types of unique features. Learning which ones pose a threat and how to deal with them may just give you the edge you need to retrieve the Orb.
The standard terrain tile is simple floor, which may change color depending on the branch but is always safe to walk on... unless it conceals a trap.
Other types of terrain are as follows.
Shallow water is a passable terrain type that may hide water-based creatures, such as swamp worms, big fishes, or electrical eels; if you notice any disturbances in the water that's a sign of a monster lying in ambush for you. Wading through shallow water is slow, and causes several negative effects such as reduced to-hit bonuses when fighting, unless you happen to be a Merfolk.
Deep water is an impassible terrain that may hide water-based creatures (same as above, but without disturbances). It can be passed with Levitation or Flight, by being a Merfolk or Octopode, or by taking on Ice Form. Falling into deep water because your Levitation/Flight expired, or because you were confused, can be fatal unless you're lucky and can scramble out of it. The game will not let you move onto deep water unless you can safely pass it; if you are confused and next to deep water, you will at least get a warning when trying to move (and possibly falling in). Monsters can fall in when confused, but in 0.9, the smarter monsters will avoid this by staying put.
Lava is an impassable terrain that may hide creatures that cast fire-based projectiles and usually have fire resistance. You cannot walk onto lava tiles without Levitation or Flight, and falling into lava can be fatal, unless you're lucky and can scramble out of it - although you'll suffer a lot more damage than if you'd scrambled out of deep water. As with deep water, the game will try to protect you from stumbling into lava, but the less intelligent monsters can be confused to fall in.
Lava floor tiles also create black smoke. Two tiles of black smoke block line of sight.
Walls create boundaries and define the Dungeon's structure.
The standard rock from which the dungeon is carved. Wands of digging can remove this terrain, as well as a handful of other spells and invocations, such as Dig or Shatter. It can also be traversed via the Passwall spell. Lightning and sparks can be bounced off them.
Translucent rock wall
Translucent rock walls are transparent but still as impassable as rock walls. They can be removed using the same means as rock walls. Some translucent rock walls make up vaults containing high-level monsters or valuable items: open them at your own risk. Lightning and sparks can be bounced off them. You can't blink through  or apport items through translucent rock walls.
Green crystal wall
These reflect cold and flame the way ordinary walls reflect lightning. Immune to digging and disintegration, but even easier to destroy with LRD or Shatter than rock walls. Destroying them with LRD will create large explosions, much larger than any other type of wall: be sure to stand a safe distance away. Green crystal normally appears in vaults rather than the regular dungeon, except in some special locations.
Harder than rock, stone walls can only be destroyed using high-powered Lee's Rapid Deconstruction or Shatter. Stone vaults are common throughout the Dungeon. Many unique, non-random structures are made from stone - orcs and gnolls have been known to create fortresses out of stone, complete with moat and drawbridge! Lightning and sparks can be bounced off them.
Translucent stone wall
The translucent equivalent of stone walls, these are just as difficult to remove as stone walls but still let you see right through them.
Even more resistant than stone, metal walls are nigh-indestructible; the only ways to affect them are to corrupt the very fabric of space near them, an ability that only followers of Lugonu the Unformed possess, or to have LRD/Shatter at nearly full power. They absorb lightning and sparks, rather than reflecting them.
"Unnaturally hard" wall
These are walls which cannot be destroyed by any means at all. This includes Lugonu's Corruption, full-powered LRD/Shatter, wands of digging, monsters such as boring beetles... nothing can get rid of them. They are generally used for places where the structure of the dungeon is important, such as Zot:5 and the edge of the map.
Even in the Dungeon, florae find a way to flourish under Fedhas Madash's guidance. Unless you follow Fedhas Madash, all florae will block your passage, and you will not be able to fire missiles through some of them. If you have not yet learned Fighting, or any given melee weapon, you can attack florae to direct some experience to the new skill, and eventually learn it when you go back to killing real monsters. (Before 0.9, you could gain skill level 1 of Fighting or weapon skills solely by fighting florae.) All of the forms listed here are passive, and cannot attack the player in any way.
Bushes have a ton of AC but are vulnerable to fire. They do not block line of sight unless there are two of them in a row, nor do they block missiles, making them perfect cover for pelting monsters from afar.
The basic fungus is immobile and has no defense nor means of attack. They can be attacked in all the usual ways. LOS is similar to bushes, except that they block (and suffer from) missiles. A shorter-lived variation shows up as the toadstools that grow on corpses. See also the Ballistomycete, which looks similar but is potentially far more hazardous.
Similar to fungi in all ways, but much tougher.
Unlike bushes and fungi, trees are effectively a type of wall. They can be set afire with spells and the like (watch out for forest fires), but the game won't let you attack them with weapons -- "You swing at nothing". Oddly, they block blinking in the same way as transparent rock walls -- try it with a scroll of blinking sometime.
Portals are temporary gateways to special dungeon branches. See the Portals page for more details.
Vaults are large, pre-fabricated dungeon features that can contain monsters, items, traps or other threats and rewards. See the Vault page for more details.