The endoplasmic reticulum is a series of membranes that carries protiens from the nucleous to any other place in the cell.
The endoplasmic reticulum was designed to carry protiens from the nucleus to the various organelles in the cell. Though some cells can produce their own protiens (such as the mitochondria and chloroplasts), they still need instruction from the nucleous itself. This is where the endoplasmic reticulum comes in. After the nucleous manufactures the protiens, it sends them along their way to the organells by way of this membrane.
The ribosomes, anchored along the length of the membrane, also produce protiens that the Endoplasmic Reticulum carries.
Scattered throughout the cell, always connect to the cell membrane.