Above is the standard 'gentleman's' knot; simple, nothing fancy, learnt at Nanny's knee. This would be spotted at Boodle's, the Member's stand at Lords, and at Smith's Lawn.
This knot is more triangular, and is called the 'Windsor' knot. Favoured by young men who consider themselves stylish and cool; it will be seen on junior bank clerks, aspiring estate agents, and others of that ilk (often worn with 'swept away' collars).
And thirdly, this rather unpleasant FAT knot fashioned at the very end of the tie (often much lower than above) is exclusively used by 14 year old rebellious schoolboys, or (if the tie is white) by Bridegrooms at their charming Chavdom weddings.
There are, of course, other knots, but it's worth knowing what secrets these three particularly common ones are telling you about their wearer.