A compound light microscope has a resolution of about 200nm. A light microscope will allow you to view cells that are alive, fixed or stained.
Bacteria (shown above) is an example of one of the smallest cells that can be seen with a light microscope. Animal and plant cells can be viewed with a light microscope as well.
In order to view small molecules or viruses, a transmission electron or scanning electron microscope is needed. These types of microscopes have a limit of resolution down to 2nm.