++ Good overview. A couple of additional points.

With Revision 004030, they've added a new separator, the Repetition Separator character, which is located at ISA11. ISA11 is the 11th element in the ISA segment. You'll see that terminology a lot. GS02 (the second element in the GS), for example, is the sender's Application ID and GS03 is the receiver's Application ID. You can split a given segment up into elements using the Element Separator, which just happens to be the 4th character of the ISA segment.

The two important separators are the segment terminator, with is the 106th character of the ISA (the ISA is the only fixed size segment... hmmm... maybe IEA is fixed size too) and the element separator is the 4th character. The component separator is the 105th character of the ISA, but it's rarely used (not used at all in 850 document IIRC). I don't know where the Repetition separator is used, but it must be even rarer than the Component separator as I've never seen it in use.

The ISA/GS segments contain information pertaining to addressing of the document and other header type information like EDI version and date/time stamps, etc.

If you find standards documents for your partner, these will typically explain what the standards are for what goes in between the ST/SE pairs.

Hope this helps.