what they are
The everyday terms people use to describe what they are searching for. "Trainers" for instance.
Short Tailed Keywords
Single terms are highly competitive and also not very effecient, if you only sell women's shoes for instance. So better to use "Women's trainers."
Long Tailed Keywords
A much less competitive term and easy to be specific to your exact customers. "Women's trainers in London" for instance ensures that you get exactly the kind of visitors you want and you are not competing with companies from another city.