If I remember this correctly, they behave the same for backwards compatibility. I'm not sure when FUNCTIONS were added to Informix, but, if memory serves me correctly, PROCEDURES pre-date FUNCTIONS.
Read the IBM Informix Guide to SQL: Syntax manual for the official position on the differences between FUNCTIONS and PROCEDURES. There are sections in the IDS v9.4 and IDS v10 manuals that describe "Relationships Between Routines, Functions, and Procedures" and "Using CREATE PROCEDURE Versus CREATE FUNCTION". You may find the same information in earlier manuals; I haven't looked to be able to say that these sections are in earlier versions of the documentation.