Pheromones were defined in 1959 by Karlson and Luscher from the Max Planck Institute for Biochemistry in Munich, Germany, as “Substances which are secreted to the outside by an individual and received by a second individual of the same species, in which they release a specific reaction, for example, a definite behavior or a developmental process.”

Thus they are, in a sense, like 'external hormones'.

Extensive research has found that they play crucial roles in regulating the behaviour of a very wide range of species - including mammals, reptiles, fish, molluscs, insects, bacteria etc etc - and even plants.

In a similar way to hormones, they play extremely varied roles - such as initiating grouping behaviour, setting pathways for navigating, as alarm signals, for attracting mates, etc etc.

It's known that many mammals are strongly affected by pheromones, some of which attract mates and encourage mating behaviour. This aspect has been exploited by, for example, pig farmers who use BoarMate* to regulate mating in their animals.

Such possibilities have led to very intense research efforts over several decades in the search for similar chemicals which could have the same effects in humans.

Though it has been found that humans can be (marginally) affected by some odours from other people, to date, no human odour classifiable as a pheromone has ever been chemically isolated in laboratory studies.

[…] if we are to find human pheromones, we need to treat ourselves as if we were a newly discovered mammal, and use the rigorous methods already proven successful in pheromone research on other species.

[…]

We can anticipate finding human pheromones on evolutionary grounds, because we are mammals but it is possible that we have lost responses to them over evolutionary time due to a lack of selection pressure. ^{[ our emphasis ]}

Source : The search for human pheromones: the lost decades and the necessity of returning to first principles Proceedings of the Royal Society B, 2014

*Note : Some researchers doubt that BoarMate actually works as a pheromone, likening it more to a 'perfume' for pigs..