Regarded as a plain category, is the category whose objects are topological spaces and whose morphisms are continuous maps.
For purposes of homotopy theory, one may want to use instead a category of nice topological spaces or a nice category of spaces.
More generally, Top may denote the archetypical (infinity,1)-category that is the archetypical (infinity,1)-topos. As such, Top is also the archetypical homotopy theory.
In this incarnation Top (the nice version) is equivalent to Simp Set and ∞-Grpd.