WoW:API for

Native Lua statement:

for [variable] = [lowvalue], [highvalue],[step] do [statements] end

Executes 'statements' for each value of 'variable' from 'lowvalue' to 'highvalue', at a step value of 'step' inclusive. Other syntax options yet unknown.

for [key],[value] in pairs([table]) do [statements] end

Executes 'statements' for each element of the 'table'. 'key' contains element key (index), 'value' contains element value.

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Examples:

 /script for i=1,10 do DEFAULT_CHAT_FRAME:AddMessage(format("%d",i));end

More interesting example: (remember, all one line)

 /script for i=1,5 do
   name,description,standingID,barValue,atWarWith,canToggleAtWar,isHeader,isCollapsed=GetFactionInfo(i);
   DEFAULT_CHAT_FRAME:AddMessage(format("%s %d/%d",name,barValue*21000,21000));
 end

A few examples using the newly found <step> value:

/script for i=10,1,-1 do DEFAULT_CHAT_FRAME:AddMessage(format("%d",i));end

Will count from 10 to 1.

/script for i=10,0,-2 do DEFAULT_CHAT_FRAME:AddMessage(format("%d",i));end

Will count from 10 to 0 skipping every other number.

/script for i=2,10,2 do DEFAULT_CHAT_FRAME:AddMessage(format("%d",i));end

Will count even numbers from 2 to 10.

Examples for the 'in' version of the syntax:

/script myTable = {"a","b","c","d"};
for k,v in pairs(myTable) do
     DEFAULT_CHAT_FRAME:AddMessage("The value of index "..k.." is "..v);
end;

Output:

The value of index 1 is a
The value of index 2 is b
The value of index 3 is c
The value of index 4 is d