Most viruses consist of four parts, the core, the capsid, the matrix, and the envelope.

The core is located in the center of the virus and contains the virus’ DNA or RNA wrapped together with some proteins. The capsid is made of protein and forms a shell around the core. The matrix forms a layer between the capsid and the envelope, while the envelope consists of lipids stolen from the cell membrane of the host cell that the virus is growing in. The envelope also contains proteins (mostly virus proteins). Viruses exist in just about every variation possible on this theme. Some viruses do not have a capsid for example, while others do not have an envelope. See the figures for examples.

In addition to a protein capsid many viruses such as HIV virus are able to envelope themselves in a modified form of one of the cell membranes- the outer membrane surrounding an infected host cell, or from internal membranes such as nuclear membrane or endoplasmic reticulum - thus gaining an outer lipid bilayer known as a viral envelope. This membrane is studded with proteins coded for by the viral genome and host genome; however the lipid membrane itself and any carbohydrates present are entirely host-coded. The Influenza virus and HIV use this strategy.

The viral envelope can give a virion a few distinct advantages over other capsid-only virions, such as protection from enzymes and certain chemicals. The proteins in it can include glycoproteins functioning as receptor molecules, allowing host cells to recognise and bind these virions, resulting in the possible uptake of the virion into the cell. Most enveloped viruses are dependent upon the envelope for infectivity.