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.
An endoplasmic reticulum (ER) in a cell is a system of membranes
which is the site of manufacture of proteins and lipids.
The "rough" component of the term "Rough Endoplasmic
Reticulum" or "RER" indicates that
there are ribosomes attached
to the surfaces of the endoplasmic reticulum.
See the Revision page about Cell
Structure for further information including a diagram of the
structure of a basic cell including nuclear pores, organelles, etc..