I quite like this simple analogy, though it is from an American.
//RICH HALL, a US stand-up comic, explained: when you go to work in the morning, if your name is on the front of the building, you're upper class; if your name is on your desk, you're middle class; and if your name is on your shirt, you're working class.//
On the 5-class socio-economic model (see middle column of my link), I've always thought of classes 1, 2 and 3 as being 'middle class', with classes 4 and 5 being 'working class':
http://www.upl.co/uploads/Class1586794153.jpg
However I've known many people working in 'the professions' (such as teachers) who prefer to see themselves as 'working class'.
Historically the working classes were those who worked in physica labour for others, This also included trades that serviced the working classes, that would own their own business such as Shopkeepers, plumbers,blacksmiths etc. Middle Classes would make a living using their brains rather than brawn -teachers bank managers etc, and Upper classes were old money landed gentry.
This has all changed as far as I'm concerned although not into a classless society far from it. Its lazy journalism, or perhaps political correctness to refer to 'The working class(es) when really they are referring to low paid workers with no job security. Maybe we should find another word. My 19 year old son has no concept of working/middle/upper class structures and I'l like this generation to keep it that way.
Having lived in a Society (North America) that has no class system -save for celebrity but anyone can become a celebrity- its very refreshing.
Most of the working-class i know have never done a days work in their lives and have no intention of ever doing so.The middle class are the new working class.The old-style working class are a new different class altogether.
If you are British and have no idea that the British class system is very much alive and kicking, then you are probably Middle-Class and complacent - because the Middle-Classes (post-World War II) have done very well... so far.