Blues can be categorized into a number of genres. There are also genres of music that are not blues but which can be described as blues-like or bluesy. What may also be called blues is the actual chord structure of a piece, which goes through a standard chord progression, called the blues chord progression, containing the 3 basics chords: I, IV and V, which means the first, the fourth and the fifth degree.
Genres of blues 
Blues-like genres 
There are several genres that are typically urban in origin, simple in instrumentation and featuring plaintive, melancholy vocals that emphasize the singer's poor luck and unfortunate circumstances. Anthropologist Joaquim Reis de Brito described the phenomenon this way:
Thus, if we take together the Fado of Lisbon, the Tango of Buenos-Aires and the Rembetika of Athens, we will note firstly that all of them emerged a little before or after the middle of the 19th century in poor districts of the big port cities of the nascent industry, attracting people from the country or from abroad, and who were confined to a marginal existence. And if we look for other parallels in the development of these urban popular cultures, we will find them again: first, their obscure and repressed beginnings, then their discovery and appropriation by elements of the higher social classes, later their acceptance and admission by the establishment (often after their success outside of the native land) before ending as a subject of tourist exploring the caves".
Note that not all of the characteristics above are common to all the genres compared to blues, and not all are true of the blues itself.