Hydrogen fluoride is a molecule consisting of one hydrogen atom and one fluorine atom. There is a relatively strong ionic bond formed between the two.
Hydrogen has one electron more in its electronic configuration than the number of electrons that would ensure a stable electron configuration. Fluorine on the other hand is short of one electron to achieve a stable electronic configuration. When hydrogen fluoride is formed one electron moves from the electron shell of hydrogen to that of fluorine. Both the atoms have a stable electronic configuration but the hydrogen atom now has a single positive charge and the fluorine atom has a single negative charge. This attracts the two and they form one hydrogen fluoride molecule.
Hydrogen fluoride molecules are also bound to each other by hydrogen bonds which affects their chemical properties in many ways.