Enveloped viruses surround themselves with plasma membrane taken from a host cell during the release stage of viral replication.  Between the virus and the envelope there are viral matrix proteins.  The envelope is modified during replication by the addition of viral glycoproteins, collectively called "spikes".

Viral spikes are the structures that enable enveloped virions to be released or "bud" out of the host cells and bind to specific cell markers on new host cells.  Examples include neuraminidase (budding factor) and hemagluttinin (attachment factor) proteins on the envelope of influenza viruses.  Spikes can also be found on nonenveloped viruses, attached directly to the nucleocapsid, as in the adenoviruses.