The Skills/Traits Window
There were skill trees in Diablo III during the first few years of development; they were seen in the playable demos at Blizzcon 2008 and 2009. During 2010 though, the D3 Team reworked the skill interface. They removed all passive skills turning them into traits which were moved to a list in their own display window. Active skills were all that remained in the skill tree, which also became a list.
Prerequisites
The only prerequisite to use a skill now is meeting a minimum Clvl requirement. All skills are arranged in tiers at Clvl 2, 3, 6, 10, 14, 20, and 26, and players can choose any skills from any tier to use, so long as they don't exceed 7 total skills. Respecs are readily available, making it easy to remove points from unwanted skills as higher level, better skills become available.
The image to the left is blurry and hard to read, but it's the best and only view of the new, Blizzcon 2010 skill/trait window. The tabs at the top switch between skills and traits, and each one shows a long list that must be scrolled down to read entirely.
Atop the window, just below the Skills tab, is a number "0" which shows how many skill points this character has available to invest. Once she levels up, that number will go to 1, and the "locked" skills further down the list will unlock and show their icons, names and descriptions. A character with a skill point views the available skills, clicks a button to learn a new skill, then when the interface expands by a window to the right, the skill may be chosen. Once it's got a point it's automatically added to the active skills, and its icon appears on the 1-4 hotkeys on the belt interface.