Generally speaking, an uncursed item is considered to be "normal", and most in-game items will be generated uncursed. Blessed items typically are more powerful and have extra bonuses on top of their usual effect, and cursed items are generally bad and have bad effects. See the Blessed and Cursed pages for more details on specific effects.
Every item in the game has a B/U/C status; however, it is not generally known by the player unless they have the Detect item status skill, use a co-aligned altar to identify items, or read a blessed scroll of identify. Also, the Appraising skill will, over time, hint at the B/U/C status of unidentified non-standard equipment (including tool-slot items and excluding wands) as good/fair/mediocre, respectively.
As of 1.2.0 prerelease 11, the probability of an item having a specific B/U/C status appears to be 80% uncursed, 10% cursed, and 10% blessed.
The only exception are gold pieces, which appear to be exempt from divine influences, and are never shown to have a B/U/C status. However, it has been discovered that although a status is not shown, in ADOM versions prior to 1.2.0 it is possible for gold to be cursed. See the Gold page for details.