As Webster says, literally a hundred tref or settlements and therefore exactly analagous to the Anglo-Saxon hundred. Although Webster's statement "written also kantry" must be taken as a reference to Old Welsh. (For one thing there is no letter 'k' in the modern Welsh alphabet.
According to Giraldus Cambriensis in his Description of Wales, written in the twelfth century said that "Wales contains in all fifty-four cantrefs" and went on to add that "South Wales contains twenty-nine cantrefs; North Wales, twelve; Powys, six: many of which are at this time in the possession of the English and Franks." (Which of course, leaves seven unaccounted for, Giraldus was never strong on detail.)
Each cantref consisted of two cwmwds or commotes each of which therefore consisted of fifty trefs. This however represented a theoretical model as derived from the Welsh Laws and the actual political geography on the ground was often quite different.
However despite Giraldus' assertions most sources seem to indicate that Wales consisted of forty-eight cantrefi (To apply the strict Welsh plural, but cantrefs if you prefer.) the names of which are given below, divided between the four major political power centres of Medieval Wales.
The list below should serve as a general guide as to which cantref belonged to which kingdom, bearing in mind that over seven hundred years (550 to 1282) boundaries shifted and ideas about what might o might not constitute a cantref may well have changed.
1. Gwynedd - North Wales
A. Ynys Mon (Anglesey)
B. Gwynedd Uwch Conwy
C. Perfeddwlad or Gwynedd Is Conwy
The core of Gwynedd was Ynys Mon and Gwynedd Uwch Conwy, with the Perfeddwlad to the east representing a territory that it would traditionally seek to dominate in periods of strength, whilst in moments of weakness southern cantrefi such as Ardudwy and Meironydd were targets for Powys.
2. Powys - Central Wales
A. Northern Powys - Powys Fadog
B. Central Powys - Powys Wenwynwyn
C. Southern Powys - Rhwng Gwy a Hafren
Note that the division between Powys Fadog and Powys Wenwynwyn wasn't made until the eleventh century but the distinction between the two general areas helps make sense of an otherwise long list. Much of Powys represented territory that Gwynedd both coveted and acquired at various times.
3. Deheubarth - South-West Wales
A. Ceredigion
B. Dyfed
C. Ystrad Tywi
Deheubarth was a creation of the mid tenth century, both Ceredigion and Dyfed had histories as independent kingdoms before that, with and without the attachment to Ystrad Tywi.
4. South West Wales
A. Morgannwg or Glywysing
B. Gwent
Although both Morgannwg and Gwent did exist as separate kingdoms at various times the two were generally united under one particular dynasty or another.
Sourced from general reading such as;
John Davies A History of Wales (Allen Lane, 1993)
Kari Mundi The Welsh Kings (Tempus, 2000)
and some specific sources;
Welsh Place Names at the Wales-Catalonia Website at
http://www.estelnet.com/catalunyacymru/catala/enwau_lleoedd_llyfr_4e.htm
and the rather fuzzy but still useful Medieval Wales map at
http://www.webexcel.ndirect.co.uk/gwarnant/hanes/maps/map.htm