Slum Housing or Squatting

The Arabic name of the United Nations' "Interregional Seminar on the Improvement of Slums and Uncontrolled Settlements" is:
"الحلقة الدراسية الأقاليمية المعنية بتحسين أحوال الأحياء الفقيرة والمستوطنات العشوائية "

This is a common term, and one that is used in UN documents, although other terms suggested above (random, unplanned) are also used.

Please see references below:

From the OECD Glossary of Statistical Terms:

"Definition:
Informal settlements are:

1. areas where groups of housing units have been constructed on land that the occupants have no legal claim to, or occupy illegally;

2. unplanned settlements and areas where housing is not in compliance with current planning and building regulations (unauthorized housing).

Source Publication:
Glossary of Environment Statistics, Studies in Methods, Series F, No. 67, United Nations, New York, 1997" http://stats.oecd.org/glossary/detail.asp?ID=1351

From a United Nations Economic Commission for Europe document "Informal settlements are a wide-ranging phenomenon: they encompass a variety of housing practices and experiences that do not comply with established norms and standards and have negative implications for urban and social sustainability. The term can, for example, refer both to dilapidated, rundown or substandard housing estates and to good-quality residential developments that violate certain rights or regulations (e.g. on public land, or causing environmental degradation)."
http://www.unece.org/press/pr2008/08env_p15e.htm

"Informal settlements (often referred to as squatter settlements or shanty towns) are dense settlements comprising communities housed in self constructed shelters under conditions of informal or traditional land tenure. They are common features of developing countries and are typically the product of an urgent need for shelter by the urban poor. As such they are characterised by a dense proliferation of small, make-shift shelters built from diverse materials, degradation of the local ecosystem and by severe social problems. " http://www.sli.unimelb.edu.au/informal/inform_set.html

And 161,000 more Google hits.

http://dictionary.reference.com/browse/shanty

How about 'ghetto', refering to a highly populated area, even though it is mainly used for ethnic minorities, but may fit depending on your context. I highly agree with 'slums' anyway, as it fits perfectly with this kind of extremely poor neighborhood.

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Note added at 23 hrs (2008-11-02 09:29:02 GMT)
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The "UNTERM" glossary uses slums
for مساكن متخلفة أو فقيرة

http://unterm.un.org/dgaacs/unterm.nsf/0/$searchForm?SearchView&Seq=1

http://unterm.un.org/dgaacs/unterm.nsf/WebView?SearchView&Qu...

it seems that "settlements" is mostly used for مستوطنات، which does not apply to the case of العشوائيات at least here in Egypt

http://unterm.un.org/dgaacs/unterm.nsf/WebView?SearchView&Qu...

"Milestone", I personally believe that it depends on your docuement, whether it is formal or informal, and in what context.
Anyway UNterm is very useful to browse through in such cases