ITU is an agency of the United Nations (but is older than them); I'm not sure if anyone controls ISO, but both are ruled by national bodies representing the countries of the world in some fashion.
The IETF, the Internet Engineering Task Force, is a volunteer organization that is currently not owned by anybody, and recognizes only individuals who show up in meetings or mailing lists in its standardization processes.
A few years back, this mattered to a lot of people; the ISO stuff was "standards", while Internet stuff was "just something slapped together by A. Random Group"; nowadays the IETF is recognized by both ISO and ITU as a valid standardization body, with liaison agreements and cross memberships and all that. Besides, the IETF got a hefty boost to its prestige when it turned out that the IETF-defined Internet protocols could be used to build a global, million-node, useful network, while the similar protocols defined by ISO simply aren't used in that many places, and definitely not as a global network.
(The ITU shows a little better here, having a global working telephone network with a somewhat larger number of users, and a global mostly interconnected X.25 network - but somehow, none of these is getting the press that the Internet gets these days)
ISO/ITU standards cost money, and are copyrighted with statements that make putting them on the photocopier to underscore a debate point a debatable practice.
Since the art of selling copyrighted, electronically available information is still in its infancy, the chances of getting electronic copies of the standards for purposes like quoting them or trying to compile the ASN.1 in them are also quite small.
The IETF, on the other hand, has always declared that the RFCs defining their standards must be freely available over the Net, and that anyone can copy, reprint, include, quote from or generally (ab)use the documents at any time. This makes starting, for instance, an implementation project for an IETF standard as a hobby project a very cheap proposition, while a similar effort on an ITU document would take a significant up-front investment if the documents were not already available in the organization.
The situation is improving; the ITU had a lot of their standards freely available from their Gopher/Web server in the spring of 1995; however, starting in July the documents are available only to subscribers; subscription for non-ITU members costs SFR 3.200,- per access account per year. See the ITU Web for details.