Players will come across two types of dungeons while playing World of Warcraft: micro dungeons and world dungeons. There are more than a hundred micro dungeons throughout the world of Azeroth, ranging in relative size from small to quite big. Micro-dungeon locations will include tombs, haunted mines, ice caves, and sunken ships to name a few. The transition into and out of these dungeons will be seamless, and you'll be able to run into other players anywhere along the way.

Article Courtesy of BLizzard.
More at http://www.blizzard.com/wow/townhall/dungeons.shtml