OB stars are hot, massive stars of spectral types O or early-type B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation. This radiation rapidly ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region or Strömgren sphere.
In lists of spectra the "spectrum of OB" refers to "unknown, but belonging to an OB association so thus of early type".