A cell is the basic unit of living organisms (both plants and animals)
that is capable of reproducing itself exactly. That is, there are
components within cells (organelles), but they do not reproduce themselves
independently of the cell(s) of which they are a part.
The nuclear membrane separates the nucleus and the nucleolus from
the rest of the contents of the cell.
See the Revision page about Cell
Structure for further information including a diagram of the
structure of a basic cell including the nuclear membrane
and all the organelles.