The term Counties of England can refer to several different sets, but the main distinction is between the administrative and historic.
Administrative counties of England (called 'administrative counties' by the 1888 local government act, but 'counties' by later legislation). These are the legal entities used for administration. In the last two centuries, they have been reformed three times - in 1880s, in the 1970s, and in the 1990s, to create the London County Council, the metropolitan counties and the unitary authorities. There have also been a number of smaller changes within the this time span.
The Traditional counties of England (called 'counties', 'historic counties' or 'traditional counties'). These at one time were administrative entities at some point between 1133 (when Carlisle was transferred from County Durham, which the advocates of historic counties accept), and 1373 (when Bristol was made an independent county, which they do not accept). The supporters state they are real existing entities not changed by successive local government reforms, although some accept certain minor border changes, notably the reversion of exclaves to their host territory. They claim that not only is it right and proper to speak of Westminster as being in Middlesex, that it is wrong to speak of it as being formerly part of it, since the county of this name was never abolished.