It depends. If the character has no way of estimating the difficulty, I don't tell the player.
If the character has some general idea of the difficulty, I typically tell them how much they pass or fail by after the roll.
If the character has a very good idea of the difficulty, such as if he's picking a lock, tumbling, or appraising, I just say "Roll me a DC X check in Y." There's no point spending two minutes trying to flavorfully describe how difficult something will be when there's already a number to equate it to (given that the players are familiar with those numbers already). If I tell the Dwarf to make a DC 30 jump check, his character knows that this is a dang hard jump. I don't need to start going over wind conditions, jump distance, how good of a running start he can get, how strong he is, how much experience he has jumping, similar jumps he's made in the past, his character's depth perception, the surface he's jumping from, the area spells going on in the area, how motivated or desperate the character is based on the situation, the character's ability to grab the ledge if he just barely misses, magical bonuses, etc.