The cells of eukaryotic organisms are elaborately subdivided into functionally-distinct membrane-bound compartments.
Some major constituents of eukaryotic cells are: extracellular space, plasma membrane, cytoplasm, nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum (ER), peroxisome, vacuoles, cytoskeleton, nucleoplasm, nucleolus, nuclear matrix and ribosomes.
The most common localizations referred to include the cytoplasm, the cytoplasmic membrane (also referred to as the inner membrane in Gram-negative bacteria), the cell wall (which is usually thicker in Gram-positive bacteria) and the extracellular environment.
[1] The experimentally determined subcellular locations of proteins can be found in UniProtKB, Compartments, and in a few more specialized resources, such as the lactic acid bacterial secretome database.
Proteome Analyst is a freely available web server and online toolkit for predicting protein subcellular localization.