Intrinsic proteins, aka 'integral ' proteins, are bound within the plasma membrane of a cell. All integral proteins have at least one region called the 'transmembrane domain ', which is composed of mostly all hydrophobic amino acids, and thus can penetrate the lipid bilayer of the plasma membrane. Many integral proteins have several transmembrane domains that snake in and out of the plasma membrane, with hydrophilic domains protruding both into the cytoplasm of the cell, and the extracellular environment. The strict definition of what makes a protein integral, is that the plasma membrane must be disrupted for the protein to be released.
Extrinsic proteins are also called 'peripheral ' proteins. These are also associated with membranes, but they do not penetrate the mebrane itself. They are usually bound to integral proteins by non-covalent bonds, and can therefore be extracted from the membrane without destroying it (using some kind of chemical wash). There are also 'anchoring ' proteins which do actually penetrate the membrane, but only slightly. Many scientists just consider these a variant of peripheral proteins , because they too can be extracted from membranes without disrupting the membrane. Other scientists, however, like to keep them in a separate category to needlessly complicate as many things as possible.
