Undergound stations outside Zone 6 were previously in zones A to D but on January 2nd 2008 previous Zones A, B, C and D become Zones 7 (A & B), 8 (C) and 9 (D). The boundary of the outermost "conventional" London travel zone (Zone 6) is more or less coterminous with the GLA (Greater London Authority) area, (i.e. 'Greater London' or the 33 London Boroughs), whereas Zones 7,8 and 9 are in Hertfordshire and Buckinghamshire. Since fare levels are affected by the level of local authority subsidy it was necessary to make separate arrangements for those outlying zones.
A useful Wikipedia page about Zones 7 to 9 here
http://en.wikipedia.org/wiki/Travelcard_Zones_7-9Regarding the line beyond Amersham, originally the Metropolitan Railway ran services all the way to Verney Junction, past Aylesbury. In 1899 the Great Central Railway was projected southwards to London, running over the Met tracks from Verney Junction to Harrow On The Hill, thereafter the GC ran on its own tracks alongside the Met to its terminus at Marylebone.
In 1906 the shared section between Harrow and Verney Junction was leased to a joint committee of the GC and Met railways, which operated the line for 5 years each alternately. This arrangement lasted until 1933 when the Met was taken over by the London Passenger Transport Board and absorbed into what became the "London Underground".
Until 1961 the Met was not electrified beyond Harrow On the Hill, and electric locomotives hauled trains of conventional carriages from the latter station to Aylesbury and Chesham. In 1961 electrification was extended to Amersham and Chesham and the line beyond Amersham handed over entirely to British Railways as it then was, the predecessor of Chiltern Railways.
1. Preserved Metropolitan Railway electric locomotive, "Sarah Siddons", used up to 1961
2. Metropolitan Line A60 type train, used since 1961, and soon to be replaced. (original condition)
3. New 'S' stock to be introduced in 2009 or 2010