Doing something like a tree structure

[Matt-D]

Hello All,

since my left frage which I use for links to picture (shown in the right frame) is growing to big I would kike to structure it in something like a tree structure. I don't really know how it's called (and I'm german) so excure my if it could be found easyly.

• 1) The page (left frame) should open like this:
  A1
  A2
  A3
  A4
• 2) Then the user clicks on "A1" and it opens sub entrys
  
  A1
  -picture 1
  -picture 2
  -picture 3
  -...
  A2
  A3
  A4
• 3) If the user clicks on "A3" now I should look like this:
  
  A1
  -link to picture 1
  -link to picture 2
  -link to picture 3
  -...
  A2
  A3
  -link to picture 1
  -link to picture 2
  A4
• 4) When the user clicks on "A1" again it should look like this:
  
  A1
  A2
  A3
  -link to picture 1
  -link to picture 2
  A4
• and so on...

How do I do something like this?
Tnaks in Advance, Matt-D