Database systems do not store data in any particular order at all. Nor are they guaranteed to retrieve data in any order, except when you supply the sort order as a part of your SQL statement.
"Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson
"I have my standards. They may be low, but I have them!" - Bette Middler
"It's a book about a Spanish guy named Manual. You should read it." - Dilbert
In Informix the default order of results on one single table is decided by the order in which the rows were inserted. The primary index stores for every row the key value(s) together with a kind of serial called rowid. This rowid is a pointer to the specific row in the involved datapage and is assigned during insertion of the row.
With results of multi table queries I guess it's decided by the rowids of the dominant table and in many cases dependent of the query plan of the optimizer.